Disease; Microbial Applications; Biotransformations; Microbial Decomposition

Paper details:

Need paper written in APA format using in text citations, headings. Ensure there is an Introduction and Conclusion paragraph.

Module 5’s Home page was packed with pathogens! Choose three that surprised you most, give a brief description, and explain why you were surprised by them.

For the second part of this assignment, you will take a closer look at the HIV virus, using this document to help you define the following terms. First begin by reviewing the structure of DNA and the processes of transcription and translation.

Next address these topics on HIV:

Discuss the function of HIV protease, integrase, and reverse transcriptase.

Identify the characteristics of HIV which account for its transmission. Include the basic epidemiology of this virus and its method of transfer.

Discuss the normal function of B-lymphocytes, killer T-lymphocytes, helper T-lymphocytes, and macrophages in the immune system. Explain how HIV affects these components of a healthy immune system.

Explain the function of the complement system and do some research to investigate how the HIV virus turns the complement system against the human body. How has this lead to new treatments for this disease (Hint: Wiki or PubMed is a good place to begin your research)?

MIC 100 Module 5 Home Page

In this Module we will apply what you have learned in this course to common diseases caused by pathogens belonging to bacterial, viral, and fungal groups.

Now that we have a general idea of what pathogens are, how they work and how they identify, adhere to and invade their human host, let us review the specific features of some significant human pathogens. We begin with common bacterial infections:

GRAM POSITIVE COCCI.

Gram + cocci are non-motile bacteria occurring in a linear arrangement (Streptococci that cause strep throat) or in clusters , like bundles of grapes (Staphylococci that cause staph infections). Many species are normal inhabitants of the skin, mucous membranes and internal organs and are normally non-pathogenic, however under altered ecologic conditions (for instance after the use of broad spectrum antibiotics or in immune deficiency syndromes), they may cause opportunistic infections. The major pathogens in this group are Streptococci and Staphylococci that cause some of the most common bacterial infections in humans. Gram + infections are responsible for millions of deaths. Proper antibiotic treatment usually controls these infections, however if antibiotics are improperly used, resistant strains develop which are difficult to control. Presently the so-called “superbugs” are a major concern among epidemiologists and general practitioners. Infections with Gram+ cocci are particularly dangerous among newborns, very old patients or immunocompromized patients.

GRAM NEGATIVE COCCI. Pathogenic gram negative cocci belong to the genus Neisseria. There are only 2 important human pathogens in this group: N. meningitides that cause meningitis (meningococcus) and N. gonorrhea that cause gonorrhea (gonococcus). Neisseria are gram negative diplococcic, mucous membrane parasites that grow either anaerobically or as facultative anaerobes. Non-pathogenic Neisseria are part of the normal oral and nasopharyngeal flora. Asymptomatic carriers may harbor N. meningitides in their upper respiratory tract! Neisseria may be isolated from blood, nasopharynx, skin, cerebrospinal fluid, urethra, cervix, vulva and anus.

Meningococcus. N. meningitides may cause an assortment of infections other than meningitis, including septicemia ( a very lethal systemic infection) nasopharyngitis, septic arthritis, endocarditis, osteomyelitis and pneumonia. Nasopharyngitis is often asymptomatic but it may be the primary infectious focus leading to meningitis or septicemia. Meningococcemia (septicemia) may or may not be associated with meningitis. The clinical picture includes high fever, disseminated microthrombosis leading to petechiae, purpura and arthritis . In several instances there may be hemorrhagic necrosis of both adrenal cortices and acute adrenal insufficiency (Waterhouse-Friderichsen syndrome). Septic shock and death may ensue within a short time.

Meningitis: Meningococcal meningitis is one of the many infections of the meninges that also include pneumococci, streptococci, staphylococci and tuberculosis.

Gonococcus: N. gonorrheae is the causative agent of the most common sexually transmitted disease throughout the world: Gonorrhea. Patients with this sexually transmitted disease may harbor other pathogens in addition to the gonococcus. N. gonorrheae also affects extra genital organs causing arthritis, pharyngitis, conjunctivitis, dermatitis and proctitis. Close to 3 million Americans are infected annually. The most common pathway of infection is through sexual intercourse. Its highest incidence is between 15 and 30 years of age.

GRAM+ SPORE-FORMING BACTERIA (BACILLI AND CLOSTRIDIA)

ANTHRAX: Of all the saprophytic forms of Bacillius, only B. anthracis is as potential human pathogen, although it is mostly responsible for diseases among horses, sheep, cattle, goats and swine. Anthrax has come to the center of world attention because of its potential to be used as illegal bacteriologic weapon. B. anthracis is a non-motile4 to 8 micrometer gram positive rod that shows a central endospore only in cultures. It is a facultative anaerobe that forms large, white, opaque colonies with undulating margins in agar plates. In smears they appear as long chains. Virulent strains are capsulated. This bacillus produces an antiphagocytic polypeptide and a protein-endotoxin complex that is antigenic. The vaccine is geared towards such a molecule. Anthrax is primarily an animal disease acquired by consuming contaminated grasses, through inhalation of the spores or through skin abrasions. The spore germinates into the vegetative form in the connective tissue. Protected by its antiphagocytic capsule, the bacillus multiplies and invades lymph vessels or blood vessels. This may lead to an often fatal septicemia. Human infections include the cutaneous anthrax that gives rise to the so-called malignant pustule and the respiratory infection (Woolsorter’s disease)

CLOSTRIDIA: This gram+ spore forming bacillus populates the soil, water and plants. Many are responsible for putrefaction and play an important role in the food chain. Few species parasitize human and animal intestines. C lostridia are gram + obligate anaerobes measuring 3 to 8 micrometers and forming rods. Spores germinate in anaerobic conditions in suitable humidity; they are heat resistant. The exotoxins are the main factors of pathogenicity. C. botulinum produces 8 neurotoxins. C. perfringens secretes 11 toxins, 2 of which cause human symptoms. C. tetanii causes tetanus by producing one of the most powerful toxins known to man, tetanospasmin. One mg of this toxin is enough to kill 200 million mice! This toxin is released during lysis of the vegetative cell and acts on the central nervous system, blocking the transmission of nervous impulses. C. tetanii is part of the normal intestinal flora in up to 25% of the population and is present in the feces of horses and cattle. This bacterium is unable to germinate in the GI tract. Infections result from contaminated wounds, with fibrin, in anaerobic conditions. It may also be present in dirty needles among drug addicts. Septic deliveries or abortions are also a source of contamination. Tetanus causes convulsions and tonic paralysis of voluntary muscles leading to respiratory muscle paralysis, opisthotonos and death.

Botulism. In most cases this is not a true infection but a poisoning caused by the ingestion of preformed toxins produced in improperly refrigerated food contaminated with the bacterium. Contaminated honey may poison children. Occasionally the toxin may form in contaminated wounds.

GRAM NEGATIVE ENTERIC BACILLI (ENTEROBACTERIACEAE)

Enterobacteriaceae are a diverse group of non-sporulating rods that inhabit the intestinal tract of animals and humans. They are responsible for a significant number of clinical syndromes, ranging from very common to rare and from trivial to lethal. All of these bacteria belong to the Family Enterobacteriaceae and are classified in 12 genera, not all of which are human pathogens. Following are the genera of clinical importance:

Genus I: Escherichia sp. coli.

Genus IV: Salmonella Sp Cholerasuis, Typhi, enteritides.

Genus V: Shigella sp,: dysenteriae, flexneri, sonnei.

Genus VI: Kleibsiella sp: pneumoniae, ozaenae, oxytoca, rhinoscleromatis.

Genus VII: Enterobacter sp: Sakasaki.

Genus IX: Serratia sp.: marcescens.

Genus X: Proteus sp: mirabilis, vulgaris, rettgeri.

Genus XI: Yersinia sp: pestis, enterocolitica, pseudotuberculosis.

Many of these species thrive in nosocomial, hospital, environments promoted by the indiscriminate use of antibiotics. The general characteristics of the whole family are:

Many members of this family have been used in genetic and bacteriologic research.

E. coli is part of the normal intestinal flora in numbers from 10 million to 100 million per gram of feces. Their presence in water is an index for fecal contamination. Some strains are pathogenic. E. coli is also responsible for urinary tract infections due to fecal contamination of the urethra in females. These infections are also common among immunocompromised or malnourished patients. Among the very young or the very old this pathogen can cause potentially lethal septicemia, meningitis and pneumonia. Mortality may be as high as 75%.

Three varieties of E. coli have been identified:

Enterotoxigenic colonizes the small intestine through a pilus and produces 2 types of enterotoxins. They cause a very watery diarrhea as seen in hospitals and the so-called traveler’s diarrhea.

Enteropathogenic strains colonize the surface of the intestine. They may invade the lamina propria (intestinal lining), but rarely cause septicemia.

Enteroinvasive strains cross the epithelium and are responsible for a bloody diarrhea (Dysentery); they produce a toxin similar to Shigella.

Salmonella species. These are actively motile rods, some strains are pathogenic for humans and animals. They survive well in adverse conditions. Infections are acquired through fecal contamination of food or fomites or through consumption of contaminated dairy products or eggs. Ten out of more than 1500 serotypes are responsible for most human infections (the most common is S. enteritides typhimurium). Salmonella infections are responsible for gastroenteritis, enteric fevers (typhoid, paratyphoid, non-typhoid) or life-threatening septicemias. In the US most infections are mild and self-limiting.

Enteric fever is caused by more invasive Salmonella, weaker patients or both. Salmonella is phagocytized by macrophages, but it is not destroyed; it penetrates the intestinal mucosa, degenerates the brush border and is temporarily stored in vacuoles within epithelial cells. Later they invade the lamina propria and invade lymphatic tissue. The intestine may be re-infected through the blood and bacteria will be eliminated through stools.

Shigella. These are enteric bacteria carried by humans and transferred through fecal contamination. Shigella causes bacillary dysentery, a type of diarrhea with blood-streaked mucoid stools, severe cramping and tenesmus. After defeating the host’s natural defenses Shigella invades the colon without causing significant bacteremia. Epithelial cells are destroyed and mucosal ulceration, edema result. There is significant lymphatic tissue hyperplasia. A grayish pseudomembrane with necrotic tissue, fibrin and bacteria appears within one day. Bacteria rarely spread beyond regional lymph nodes. The incubation period is one to two days. S. dysenteriae, a species that is not common in the US, is the most aggressive and may result in a 50% mortality rate.

Yersinia. This bacteria (formerly called Pasteurella) was responsible for one of the deadliest epidemics known to mankind, the Bubonic Plague, that allegedly killed up to 40% of the population of Europe in the XIV Century. Presently in the US Y enterocolitica may be responsible for some cases of ulcerative colitis with microabscesses, pharyngitis, tonsilitis and cervical lymphadenitis. Y. enterocolitica is invasive and may cause a rare, but lethal type of dysentery with severe abdominal pain. The natural reservoir of Y. pestis are wild and domestic rats and other rodents. Bacteria are transmitted to humans through the fleas of these rodents.

Other enterobacteriaceae of clinical importance include Providencia, Enterobacter, Proteus, Kleibsiella and Serratia marcenscens. K. pneumoniae is a normal inhabitant of the human nasopharynx. It is responsible for many types of infection, including septicemia.

MYCOBACTERIA

The most important representatives of this genus include Mycobacterium tuberculosis, the etiologic agent in tuberculosis and M. leprae, the causative agent in leprosy. Other mycobacteria are collectively called “atypical”; some of these are responsible for opportunistic infections among AIDS patients. Mycobacteria belong to a small group of bacteria containing a thick and complex cell coat with waxes, including mycolic acid. Mycobacteria are gram positive, but their classification is based in their ability to resist decoloration (bleaching) with acid alcohol (acid-alcohol resistant). Ziehl and Neelsen developed a stain using carbofuscin followed by acid alcohol and methylene blue as a counter stain. The waxy coat of mycobacteria render them resistant to decoloration, therefore they retain the red dye, while other bacteria will bleach and stain blue. Mycobacteria grow slowly and require around 24 hours (instead of the usual minutes) to duplicate their population. M. tuberculosis does not produce toxins. Its virulence is due to the immune reaction (delayed hypersensitivity) elicited in the host that leads to severe chronic inflammation and tissue death. The long-term survival of mycobacteria is due in part to the inability of macrophages to destroy them and the long survival within these cells (often years). Most commonly the host’s immune system reaches a compromise with the bacteria and both coexist peacefully without damaging each other.

Tuberculosis is a chronic, granulomatous disease most often caused by M. tuberculosis and occasionally by M. bovis or other atypical mycobacteria. Its primary target are the lungs but it may also affect extrapulmonary sites such as bones, joints, digestive tract, adrenal glands, meninges, kidneys, skin, etc. This pathogen is a strict aerobe, intracellular, non-motile, slender rod measuring around 4 micrometers.

Tuberculosis is still the most important bacterial infection throughout the world. Its incidence and severity are increasing, partly due to the AIDS epidemic. TB is most often acquired through long-term exposure to respiratory droplets from infected patients. The digestive pathway is favored by M. bovis. TB bacilli are very resistant to adverse environmental conditions, and may survive for several weeks. Infections are more common in overcrowded, impoverished environments. Malnutrition, debilitating diseases and immune deficiencies increase the susceptibility. Of all the individuals infected with M. tuberculosis, only a few develop the full-blown syndrome.

Leprosy. Leprosy is an infectious disease of the skin and cutaneous membranes caused by Mycobacterium leprae. Once a common and devastating disease, leprosy has been largely controlled in the US. Most cases in this country are concentrated in a facility in Carville, Louisiana. Endemic foci are found in poor communities worldwide. The disease has low communicability. The bacteria has the lowest division rate among known pathogens.

MYCOPLASMA

Mycoplasma are a group of very small, pleomorphic bacteria without cell wall, previously thought to be viruses, because they are capable of crossing bacterial filters. They are free living and populate the soil, plants and animals. The first pathologic condition associated with mycoplasma was bovine pleuropneumonia, so for a while these organisms were called pleuropneumonia-like organisms (PPLO). The only family that infects humans is Mycoplasmataceae. The three human pathogens are: Mycoplasma pneumoniae; M. hominis and Ureaplasma ureolyticus.

SPIROCHETES

So far we have described mostly bacteria with coccal or bacillary shapes. The 3rd general type bacterial shape are the spirilla: corkscrew-shaped, slender organisms that include several important human pathogens. The two families of medical importance are the Spirillaceae that include the genera Spirillum and Campylobacter, and the Spirochaetaceae (or spirochetes) that include the genera Treponema, Borrelia and Leptospira.

Spirillaceae are motile, slender, gram negative bacteria with polar flagella. Spirochetes are long, helical bacteria that move by means of axial filaments.

An important spirochetal human disease is syphilis. Once thought to be under control, the incidence of syphilis is increasing lately.

Borrelia are large, non-motile spirochetes. They include some human pathogens causing relapsing fever and Lyme’s disease. The natural hosts are arthropods that transmit the disease to humans and other vertebrates through their bite.

Lyme’s disease is a biphasic syndrome involving cutaneous lesions and articular manifestations with occasional neurologic symptoms. It was characterized in 1972 in the town of Lyme, CT. It is caused by Borrelia burgdorferi and transmitted by a deer tick: Ixodes dammini.

An important human pathogen , similar in aspect to Campylobacter is Helicobacter pylori. This spiral gram negative rod is now blamed for the majority of cases of peptic ulcers, both gastric and duodenal.

RICKETTSIAE AND CHLAMYDIAE.

Rickettsiae and Chlamydiae are very small gram negative bacteria once thought to be viruses. They are intracellular parasites and have been successfully cultured in laboratory media. Most rickettsiae are transmitted by insect vectors, whereas Chlamydiae are transmitted from person to person.

Rickettsiae are gram negative rods measuring 0.3 to 0.7 micrometers in diameter, transmitted by ticks, fleas, lice and mites (except Coxiella burnetti, the agent for Q-fever). These bacteria reproduce in the insect vector, invade their salivary glands or remain in the gut and are released through stools and are then deposited and inoculated while they bite their mammalian hosts. Once in the connective tissue they invade vessels, attach to the lining cells and kill them. Rickettsiae are responsible for 5 types of diseases: typhus, spotted fevers, scrub typhus, Q-fever and trench fever.

Spotted fevers: The most common type in this country (Virginia, North Carolina and Tennessee) is the Rocky Mountain spotted fever, occurring mostly among children and young adults. It is caused by R. rickettsii. The most common vector is the house dog tick, whose reservoir may be in wild animals.

Chlamydiae cause the most frequent sexually transmitted disease: the so-called nongonococcal urethritis, capable of causing pelvic inflammatory disease in women, that in turn may lead to sterility or to ectopic pregnancies. Chlamydia are also responsible for ocular infections. Chlamydia are strict intracellular bacteria; they cannot be cultured in cell-free media. They have a complex life cycle: one form is responsible for intracellular multiplication and the other for the transit between the host and its cells.

Another significant Chlamydial infection is trachoma. Although rare in the US, trachoma is a leading cause of blindness throughout the world.

Viral

Viral infections: When a host cell and a virus meet there are for possible outcomes:

No infection occurs. The host cell has no viral receptors (non-specific virus)

Infection occurs, virus replicates and lyuses the cell.

The virus infects the cell but it does not replicate because an equilibrium is reached (lysogeny).

The viral genome (oncovirus) is spliced in the host cell’s DNA and causes the uncontrolled replication of the infected cell leading to a tumor. (oncogenic virus). Such cells no longer respond to the mechanisms that regulate cell division in the body.

Viruses may infect humans from person to person or through vectors. Viral infections may spread through air, water, food, through the skin or through sexual contact; such a spread is called horizontal. Vertical spread refers to viruses passed before birth through the placental barrier (congenital), after birth (post natal) or during birth (perinatal)

Selected viral disorders:

Poxviruses. These DNA viruses are the largest animal viruses. They cause cutaneous lesions in several animals including humans. Some are primarily human pathogens and some others primarily infect animals and secondarily humans. The most important subgenus of the pox family is called Vaccinia; it includes the viruses causing vaccinia, cowpox and variola. All these species cross-react. Jenner developed the first vaccine in 1779 against smallpox. These viruses contain double-stranded DNA and are brick-shaped, ranging from 200 to 300 nanometers. They are the only viruses large enough to be seen under the light microscope.

Herpes viruses. These are relatively large, cubic, enveloped DNA viruses that replicate in the infected cell’s nucleus and have a genome coding for around 50 to 80 proteins. The envelope is borrowed from the host cell. Important pathogenic human pox viruses are: Varicella zozter virus, Epstein Barr virus and herpes simplex virus. All of them have the ability to remain latent and recur.

Varicella Zoster (Chicken pox and shingles). Chickenpox is a benign childhood disease and shingles may occur later in life as a result of reactivation of the virus.

Herpes simplex virus. This is a very common source of human infections; it includes cold sores, fever blisters, keratitis (corneal infections), encephalitis (brain infections) and venereal infections. Herpes simplex is also responsible for perinatal and post natal infections. There are 2 known serotypes (identified by testing the serum of infected patients): Type 1 involves the upper body (above the waist) and Type 2 causes genital lesions. Herpes infections may be primary or may result from the reactivation of a latent virosis, traveling through the axons of both sensory and motor nerves to the skin or mucous membranes.

Adenoviruses. This group of DNA naked viruses measures 70 to 90 nm. Adenoviruses infect many animal species including humans. There are 39 human adenoviruses, some of which have a very complex structure resembling artificial satellites. Each virus has an antigenic fiber attached that causes hemagglutination (clumping of red blood cells) and is used for attachment. Adenoviruses have a tropism for mucocutaneous membranes and are commonly associated with respiratory infections, most commonly in infants and children. The only potentially fatal adenoviral infection is pneumonia. Less commonly these viruses may cause pharyngitis, epidemic keratoconjunctivitis and occasionally gastrointestinal disorders. Adenoviral infections affect each child in this country and throughout the world and account for a high percentage of all respiratory infections, particularly among hospitalized children. Adult infections are less frequent. The typical syndrome includes pharyngitis, bronchitis, croup and pneumonia. Signs and symptoms vary according to the serotype involved. The virus is identified in human kidney tissue cultures. There are 21 potentially pathogenic serotypes. Vaccines are available against serotypes 4 and 7 that often cause outbreaks among military employees.

Papillomaviruses. Large group of DNA viruses, some of which are pathogens for humans and other animal species. They belong in the family of Papovaviruses (papilloma, polyoma, vacuolating viruses). The 45 known serotypes have tropism for mucous membranes and the epidermis. The main effect of these viruses is the formation of warts:

Verruca vulgaris (common wart) that occurs anywhere in the skin or oral mucosa, particularly in the hands.

Verruca plana (flat wart) often present in the face and dorsum of the hands. This is a flat lesion.

Verruca plantaris or palmaris that affect the palms of the hands and soles of the feet.

Condyloma acuminatum (venereal wart) occurs in the penis, female genitalia, urethra, perianal region and rectum. These are soft, tan, raspberry-like masses. Venereal warts are often associated with other sexually transmitted diseases. Infections in the uterine cervix may lead to carcinomas.

Parvoviruses. These are naked, single-stranded DNA viruses measuring 18-26 nanometers. There are 2 human pathogens: Norwalk agent and parvovirus B-19. The former infects children and adults and causes vomiting, diarrhea and abdominal pain.

RNA VIRUSES

Measles. Eruptive pediatric disease caused by an enveloped spherical RNA virus belonging in the family of Paramyxoviridae.

Mumps (epidemic Parotiditis). This acute viral pediatric disorder is caused by another member of the Paramyxoviridae family. The mumps virus is spherical, enveloped and measures around 180 nm in diameter. Adult infections are less frequent and may involve the testicles (orchitis).

Rubella. This is an eruptive childhood disorder caused by an RNA virus of the Togaviridae family, spherical, around 60 nm in diameter with a lipoprotein envelope. Infections during the first trimester of pregnancy often lead to congenital malformations. The disease was originally described in Germany in 1800 (German Measles).

Influenza. This acute Respiratory infection is caused by an RNA virus of the Orthomyxoviridae family. These viruses may be spherical or filamentous and measure around 100 nm. The viral RNA consists of 8 distinct molecules; this allows considerable genetic recombination that is responsible for the numerous antigenic differences and the difficulty in creating an all-inclusive vaccine. The influenza epidemic of 1917-1918 killed around 22 million people worldwide.

Poliomyelitis. This acute viral infection predominates among children (Infantile paralysis). It affects lower motor neurons. (nerve cells located in the anterior horns of the spinal cord) and causes neurogenic atrophy of the muscles normally innervated by the infected motor neural pathways. It is caused by the poliovirus belonging to the Picornaviridae family that also includes enteroviruses. There are 3 distinctive serotypes with no cross-immunity amongst them. Most infections in the US were due to type 1 virus. Poliomyelitis was originally described in the XIX Century. Up until 1955 when the inactivated virus vaccine was first introduced, there were around 10 cases per 100,000 inhabitants. The infection has a high morbidity. The attenuated viral vaccine discovered by Salk was introduced in 1962. This lead to a sharp decrease in the incidence. Massive vaccination was possible with the oral vaccine. Presently there is an incidence of less than 0.001 per 100,000 inhabitants. Poliomyelitis is still prevalent in those countries with poor sanitation and no vaccination.

Rabies. This infection is due to an RNA virus, member of the Rhabdoviridae family, bullet-like, measuring 180 nm. In 1886 Pasteur discovered that rabies is an infectious disease caused by agents smaller than bacteria (virus). He performed the first vaccination with inactivated virus prepared from rabbit’s brain.

Yellow fever. This acute infection is caused by an RNA virus of the Togaviridae family. It is transmitted through the mosquito vector Aedes aegypti. No cases have been reported in the US since 1943. In Africa wild monkeys are a natural reservoir of the virus.

Viral encephalitis. Viral infections of the brain may be caused by several types of virus (mumps, measles, herpes, etc.) or by a primary neurotropic virus transmitted through mosquito vectors (Arboviruses) Some of these infections are endemic in some regions of the US. Most infections are mild, characterized by vomiting, headaches and neck stiffness. A few patients develop convulsions and may go into coma. Some patients end up with severe neurologic sequels. The most severe form in the US is eastern equine encephalitis, with a mortality approaching 70%; the mildest form is western equine encephalitis. Horses are the common reservoirs.

Rhinoviruses. These are a group of around 90 serotypes of picorna viruses. Rhinoviruses account for around 40% of common cold infections. This type of common cold is preceded by an incubation period of around 3 days, the symptomatic phase includes coryza, sore throat and cough with no fever. Some children may develop bronchitis. Treatment is symptomatic. The disease is self-limiting and lasts from 7 to 10 days.

Enteroviruses. These are an extensive group of viruses affecting the Digestive System. It includes 5 groups: Polioviruses (described above); Coxsackie groups A and B, echoviruses and enteroviruses. Enteroviruses infect the Digestive tract and spread to several organ systems.

Hepatitis viruses. Viruses capable of infecting the liver include DNA and RNA viruses. Although some of the viruses discussed previously may involve the liver, hepatitis viruses are a distinct group including hepatitis A, hepatitis B and hepatitis C (formerly non A-non B) Hepatitis A is an infectious, epidemic short term infection caused by a single-stranded, non enveloped RNA virus ; hepatitis B is often acquired through transfusions and has a long incubation period; it is caused by a double-stranded DNA virus.

Hepatitis A: is a liver disease caused by the hepatitis A virus (HAV). Hepatitis A can affect anyone. In the United States, hepatitis A can occur in situations ranging from isolated cases of disease to widespread epidemics.

Hepatitis B: is a serious disease caused by a virus that attacks the liver. The virus, which is called hepatitis B virus (HBV), can cause lifelong infection, cirrhosis (scarring) of the liver, liver cancer, liver failure, and death.

Hepatitis C: is a liver disease caused by the hepatitis C virus (HCV), which is found in the blood of persons who have the disease. HCV is spread by contact with the blood of an infected person.

Hepatitis D: is a liver disease caused by the hepatitis D virus (HDV), a defective virus that needs the hepatitis B virus to exist. Hepatitis D virus (HDV) is found in the blood of persons infected with the virus.

Hepatitis E: is a liver disease caused by the hepatitis E virus (HEV) transmitted in much the same way as hepatitis A virus. Hepatitis E, however, does not occur often in the United States.

Retroviruses. These are RNA viruses that possess the enzyme reverse transcriptase through which they copy their genes “backwards” into the host cell’s genome. Retroviruses are small, spherical virions surrounded by a lipid envelope. They contain genes for the synthesis of their core protein, another is the reverse transcriptase and a third gene is an integrase necessary for the incorporation of newly formed DNA into the host cell’s genome. These viruses are lymphotropic for cells with a CD-4 marker (t-helper lymphocytes). The most significant member of this group is the HIV virus, responsible for the Acquired Immune Deficiency Syndrome (AIDS).

Conclusion. The viral infections described above are only a few of the many types that affect the worldwide population. The best way to deal with viral infections is through prevention: avoiding contact with infected people, avoiding the insect vectors, safe sex, etc. Vaccination has been one of the main weapons for the control of viral disorders, unfortunately due to the genetic variability and the ability of viruses to mutate vaccines have only been produced against some viral infections. Many infections with viruses are self-limited and benign and require only symptomatic