The smallest parasites in the world are viruses and prions. They cannot be seen with conventional light microscopes because they are so small (between about 10 to 200 millimicrons across - and a millimicron is 0.000001 millimeters). Only the invention of the electron microscope allowed them to be seen and identified.
Viruses are strands of RNA or DNA with a protective protein coat. Viruses cannot reproduce on their own but can invade the cells of higher organisms. Here the virus's genetic material takes over and turns the cell into a virus production factory. The first virus to be identified was the tobacco mosaic virus in the 1930's, although prior to that a number of scientists had realised that some infectious diseases were not caused by bacteria and so suspected the existence of viruses.
Using the word existence with viruses is a bit suspect because viruses are on the borderline between life and non-life. They can only reproduce when they have invaded a cell, whether bacterial or of a higher life form. Viruses are hard to treat because they are hidden inside host cells and therefore the body's immune system does not recognise them as enemies. Antibiotics don't work either. Viruses cause a wide range of diseases including small pox, influenza, chicken pox, measles, rabies, AIDS, polio and cold sores.
While viruses are made up of DNA and RNA, prions are primarily made up of proteins. They are also parasitic and cause a number of fatal diseases. They gained notoriety when they were identified as the causative agent of Mad Cow Disease. These rogue proteins eat away at the host's nervous system. Because they do not have any genetic material, they cannot reproduce in the normal way, but instead, they fold themselves and subvert other healthy proteins.
Like viruses, prions are very hard to kill and only extreme heat and pressure will do so. Luckily they live primarily in nervous tissue so the best way to avoid them is to not eat brains. One of the diseases they cause, Kuru, was once common amongst tribes that practiced cannibalism. As cannibalism died out, so did Kuru. Mad cow disease can be prevented by not feeding cows or other herbivores destined for human consumption with the raw remains of animals.
There is still much to be learned about these, the smallest known parasites. It was once thought that the border between life and non-life was large and clear but prions and viruses show that this is not so. They do not have cells and are thousands of times smaller than bacteria. They cannot exist outside of host organisms and yet they can and do reproduce, to the detriment of their hosts.
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