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Viruses cause a number of diseases in eukaryotes. In humans, smallpox, the common cold, chickenpox,
influenza, shingles, herpes, polio, rabies, Ebola, hanta fever, and AIDS are examples of viral diseases.
Even some types of cancer -- though definitely not all -- have been linked to viruses.
Viruses themselves have no fossil record, but it is quite possible that they have left traces in the history
of life. It has been hypothesized that viruses may be responsible for some of the extinctions seen in the
fossil record (Emiliani, 1993). It was once thought by some that outbreaks of viral disease might have
been responsible for mass extinctions, such as the extinction of the dinosaurs and other life forms. This
theory is hard to test but seems unlikely, since a given virus can typically cause disease only in one
species or in a group of related species. Even a hypothetical virus that could infect and kill all
dinosaurs, 65 million years ago, could not have infected the ammonites or foraminifera that also went
extinct at the same time.
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