Emerging viral diseases such as those due to HIV, ebolavirus and hantavirus, appear regularly. Now that antibiotics effectively control most bacterial infections, viral infections pose a relatively greater and less controlled threat to human health. Some data suggest that the already broad gamut of established viral diseases soon may be expanded to include other serious human ailments such as juvenile diabetes, rheumatoid arthritis, various neurologic and immunologic disorders, and some tumors. Like other microorganisms, viruses may have played a role in the natural selection of animal species. A documented example is the natural selection of rabbits resistant to virulent myxoma virus during several epidemics deliberately induced to control the rabbit population in Australia.
Indirect evidence suggests that the same selective role was played by smallpox virus in humans.
Another possible, though unproved, mechanism by which viruses may affect evolution is by introducing viral genetic material into animal cells by mechanisms similar to those that govern gene transfer by bacteriophages. For example, genes from avirulent retrovirus integrated into genomes of chickens or mice produce resistance to reinfection by related, virulent retroviruses. The same relationship may exist for human retroviruses, since human leukemia-causing retroviruses have been reported. The nucleic acid contains the genetic information necessary to program the synthetic machinery of the host cell for viral replication. Once the viral genome has penetrated and thereby infected the host cell, virus replication mainly depends on host cell machinery for energy and synthetic requirements.