A virus is a submicroscopic, parasitic entity that consists of genetic material (either DNA or RNA) inside a protective protein coating and that can reproduce only while inside living cells of organisms. Viruses are not considered to be fully alive, but rather on the boundary between life and non-life, because they lack the systems for metabolism and reproduction that living organisms have as well as the rigid cell walls that bacteria and plants have.
This allows viruses to be extremely small, typically between 20 to 400 nanometers, and thus they are only visible with an electron microscope. By comparison, bacteria are commonly in the 400 to 5000 nanometers range, and human cells are still larger, at 10,000 to 100,000 nanometers.
This small size, together with a large variety of shapes and structures, is integral to the ability of viruses to enter host cells. Viruses enter and infect all life forms, including archaea and bacteria. Moreover, they can even infect a cell that is already infected by another virus.
More than 11,000 species of viruses are currently recognized. However, the total number could be in the millions and could even exceed the total number of species for all living organisms.
As is the case with living organisms, viruses evolve. However, they evolve much more rapidly because of their high rates of mutation, short generation spans, large population sizes, and persistent selection pressure from both host immune systems and antiviral drugs. A major factor in the high mutation rates is the absence of proofreading mechanisms for RNA replication. The rapid mutation rates add to the complexity of disease management and vaccine development.
Although viruses are often viewed solely as being harmful pathogens, they also have beneficial roles in ecosystems and in human health. One major role is contributing to evolution through promoting genetic diversity via the horizontal transfer of genes among organisms. Such transfer allows the sharing of beneficial traits that can enhance adaptation to environmental changes and thus facilitate species survival.
Another important role is helping to regulate bacterial populations in water bodies by infecting and killing specific bacteria species, thereby releasing nutrients back into the water and thus supporting a diversity of marine life.
Viruses have also become an important tool in medicine. They are used in vaccines to stimulate immune responses without causing disease, and they are employed to deliver therapeutic genes into target cells to treat genetic disorders.
Just as the deterioration of the natural environment is resulting in a loss of biodiversity, such deterioration could also be reducing the diversity of viruses because their health and survival is largely dependent on those of their host species and general environmental conditions. The consequences of the loss of virus diversity are not yet well understood.