It is possible that viruses existed even before the first cellular organisms were formed. These tiny organisms are not based on cellular structures. They have a central core of a single or double strand of either DNA or RNA, covered in a protective coat of capsid protein. They are incapable of replicating independently, producing their own energy, or making proteins. They invade cells and re-program their DNA. The capsid proteins protect the genetic materials inside the virus, and control the attachment of the virus to cell receptors.

The best-known viruses are the common cold, the flu, hepatitis, and HIV. Each infected person is a study in evolution, as the viruses work frantically to reproduce while evading the immune system of the host and any anti-viral medicine the host is taking. The challenge of successful treatment is to find a way of arresting the evolutionary process long enough to kill the virus.

Viral genomes, particularly the RNA version, mutate and recombine swiftly. Natural selection can occur within the infected cell, during the spread of virus in the body, or in the transmission from host to host. Since the survival of a virus depends largely on its ability to move from host to host, natural selection favors those viruses that are better transmitted, are less susceptible to antibodies, and are persistent. The ability to produce reactions that promote excretion (coughing, sneezing, diarrhea) is also a positive evolutionary trait. Natural selection favors less virulent strains which are more likely to leave the host mobile enough to contact other potential hosts.

The great increase in human population and mobility has given the viruses the golden opportunity to produce new serotypes. Genetic reassortment and exchange of viruses between humans and animals lead to endless variations. Lineages that are unable to infect other hosts will become extinct when the host's immune system destroys the infection, or the host dies.

Because viruses constantly move genetic material from cell to cell, they may be a factor in accelerating evolution in other life forms. Once viruses find a useful protein or gene, they can transmit it to more complex species. This process is called horizontal gene transfer (HGT), which creates many more changes in cells than the occasional random mutation. This may explain why it took 2.5 billion years for evolution to move from single-celled organisms to the first multicellular life, but the it took

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