Viruses, because of their predatory nature, have shaped the history and evolution of their hosts. Virtually all living organisams, when studied carefully, have viral parasites, and so these smallest of living entities exert significant forces upon all life forms, including themselves (1).
A virus is a small infectious agent that replicates only inside the living cells of other organisms. Viruses can infect all types of life forms, from animals and plants to bacteria and archaea (2).
Viruses depend on the host cells that they infect to reproduce. When found outside of host cells, viruses exist as a protein coat or capsid, sometimes enclosed within a membrane. The capsid encloses either DNA or RNA which codes for the virus elements. While in this form outside the cell, the virus is metabollically 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. Some viruses may remain dormant inside host cells for long periods, causing no obvious change in their host cells (a stage known as the lysogenic phase). But when a dormant virus is stimulated, it enters the lytic phase: new viruses are formed, self-assemble, and burst out of the host cell, killing the cell and going on to infect other cells. The diagram below at right shows a virus that attacks bacteria, known as the lambda bacteriophage, which measures roughly 200 nanometers (3).
Several questions about viruses.
How small is a virus?
Well, you might have the answer HERE.
Are viruses alive?
This is still a scientific debating topic. But you might have some clue from this interesting article: Are Viruses Alive?
How old is a virus?
A giant DNA virus called Pithovirus sibericum, was isolated from a >30,000-y-old radiocarbon-dated sample (4). Probably indicate how old it is.
How many viruses are there?
For this we might introduce International Committee on Taxonomy of Viruses (ICTV) . According to their report now we have at least 103 virus families. For each family we have more than 1000 species, and each species has hundreds of subtypes. So actually there is no accurate number for this question. But you might say there were about one million viruses of vertebrates.
(1) Fields Virology 5th edition, 2007
(3) ‘Introduction to the viruses’. http://www.ucmp.berkeley.edu/alllife/virus.html
(4) Legendre, Matthieu, et al. “Thirty-thousand-year-old distant relative of giant icosahedral DNA viruses with a pandoravirus morphology.” Proceedings of the National Academy of Sciences 111.11 (2014): 4274-4279.