Smaller than bacteria, viruses can grow and survive only when they are in the living cells of organisms. They multiply rapidly in warm and moist places.
Viruses are microscopic parasites, generally much smaller than bacteria. They lack the capacity to thrive and reproduce outside of a host body.
Predominantly, viruses have a reputation for being the cause of contagion. Widespread events of disease and death have no doubt bolstered such a reputation. The 2014 outbreak of Ebola in West Africa, and the 2009 H1N1/swine flu pandemic (a widespread global outbreak) likely come to mind. While such viruses certainly are wily foes for scientists and medical professionals, others of their ilk have been instrumental as research tools; furthering the understanding of basic cellular processes such as the mechanics of protein synthesis, and of viruses themselves.
Discovery
How much smaller are most viruses in comparison to bacteria? Quite a bit. With a diameter of 220 nanometers, the measles virus is about 8 times smaller than E.coli bacteria. At 45 nm, the hepatitis virus is about 40 times smaller than E.coli. For a sense of how small this is, David R. Wessner, a professor of biology at Davidson College, provides an analogy in a 2010 article published in the journal Nature Education: The polio virus, 30 nm across, is about 10,000 times smaller than a grain of salt. Such differences in size between viruses and bacteria provided the critical first clue of the former’s existence.
Toward the end of the 19th century the notion that microorganisms, especially bacteria, could cause disease was well established. However, researchers looking into a troubling disease in tobacco — the tobacco mosaic disease — were somewhat stumped as to its cause.
In an 1886 research paper titled “Concerning the Mosaic Disease of Tobacco,” Adolf Mayer, a German chemist and agricultural researcher, published the results of his extensive experiments. In particular, Mayer found that when he crushed up infected leaves and injected the noxious juice into the veins of healthy tobacco leaves it resulted in the yellowish speckling and discoloration characteristic of the disease. Mayer correctly surmised that whatever was causing tobacco mosaic disease was in the leafy juice. However, more concrete results eluded him. Mayer felt certain that whatever was causing the disease was of bacterial origin, but he was unable to isolate the disease-causing agent or identify it under a microscope. Nor could he recreate the disease by injecting healthy plants with a range of known bacteria.