The survival of viruses is a great example of natural selection and survival of the fittest. One such example is HIV. HIV reproduces very quickly (about 10 million new viruses per day. HIV lacks the proteins which repair mutations, and therefore there will be many mutations resulting from this rapid reproduction. The rapid rate of reproduction combined with the many different mutations has had a limiting effect on the effectiveness of HIV therapy using antibiotics. Without the mutations the virus would fall victim to antibiotics.

Are there other viruses which have proven difficult to treat because of mutations?

What viruses were very deadly and were eliminated because of the antibiotics?