In 1898, Friedrich Loeffler and Paul Frosch found evidence that the cause of foot-and-mouth disease in livestock was an infectious particle smaller than any bacteria. This was the first clue to the nature of viruses, genetic entities that lie somewhere in the grey area between living and non-living states.
Viruses depend on the host cells that they infect to reproduce. When found outside of host cells, viruses consist of a protein coat or capsid, sometimes enclosed in a membrane, that encloses either DNA or RNA: these can be seen in the pictures above. To the left is an electron micrograph of a cluster of influenza viruses, each about 100 nanometers (billionths of a meter) long; both membrane and protein coat are visible. On the right is a micrograph of the virus that causes tobacco mosaic disease in tobacco plants. In this state the virus is metabolically inert. 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. Eventually, the new viruses self-assemble and burst out of the host cell. The diagram below shows a virus that attacks bacteria, known as the lambda bacteriophage, which measures roughly 200 nanometers.
Viruses cause a number of diseases in eukaryotes; in humans, smallpox, influenza, polio, rabies, Ebola, and AIDS are examples of viral diseases. It has been hypothesized that viruses may be responsible for some of the extinctions seen in the fossil record (Emiliani, 1993). On the other hand, because viruses can transfer genetic material between different species of host, they are extensively used in genetic engineering. Viruses also carry out natural "genetic engineering": a virus may incorporate some genetic material from its host as it is replicating, and transfer this genetic information to a new host, even to a host unrelated to the previous host. This is known as transduction.
Emiliani, C. 1993. Extinction and viruses. BioSystems 31: 155-159.
