What is a virus?

Viruses are non-cellular organisms that are tiny, simple in structure, contain only one nucleic acid (DNA or RNA), and must parasitize and proliferate by replication in living cells.

Some scientists believe that viruses are not living organisms, but are merely fragments of DNA and RNA dependent on cellular life. They point out that viruses cannot replicate (proliferate) outside the host cell, and depend on the cell's protein-assembling machinery to carry out their functions. But much evidence suggests that viruses are not that different from other organisms.

Gustavo said, "Many organisms depend on other organisms to survive, such as bacteria that live inside cells, fungi in specialized parasitic relationships-they depend on the host to complete their life history." Viruses, he said, "do exactly the same thing."

Gustavo said the discovery of giant mimic viruses in the early 21st century also challenged conventional wisdom about the nature of viruses. "These huge viruses are different from the tiny Ebola virus, which has only seven genes, in that they are huge in size and have a huge gene pool." Some of the viruses, he said, "are about the same size in shape and genome as, if not larger than, parasitic bacteria.

Some of the huge viruses also have genes that encode key proteins in the translation process, the process by which cells read gene sequences to form proteins, and the lack of a translation machine within the viruses had been a reason to categorize them as non-living things, Gustavo said.

"That rationale has changed over time." Gustavo said, "Now viruses deserve a place in the tree of life. Apparently, viruses can't be as simple as we used to think."