The capsid encloses either DNA or RNA which codes for the virus elements. On the right is a micrograph of the virus that causes tobacco mosaic disease in tobacco plants. When it comes into contact with a host cell, a virus can insert its genetic material into its host, literally taking over the host's functions. An infected cell produces more viral protein and genetic material instead of its usual products. The diagram below at right shows a virus that attacks bacteria, known as the lambda bacteriophage , which measures roughly 200 nanometers.
Viruses cause a number of diseases in eukaryotes.
AIDS are examples of viral diseases. Viruses themselves have no fossil record, but it is quite possible that they have left traces in the history of life. 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. On the other hand, because viruses can transfer genetic material between different species of host, they are extensively used in genetic engineering.