Bacteria are the smallest animal forms capable of an independent existence. Even the largest are less than a tenth the size of a human red blood corpuscle, which itself is a mere 0.08 mm (0.003 in) across. The smallest bacteria of all are a minuscule one-fourteen-thousandth the size of the blood cell.
Yet these minute organisms are giants compared to iruses, organisms that require a host cell in which to live and reproduce. At the beginning of this century, viruses were known as 'filterable agents' because they passed through filters fine enough to trap any bacteria.. The influenza virus, for example, is about one-sixth the size of the smallest bacterium. The tobacco mosaic virus, which discolours the leaves of the tobacco plant, is 100 times smaller than the smallest bacterium, and the small known virus of all, one that causes spindle tuber disease in potatoes, is 2500 times smaller. They are only 'visible' through powerful electron microscopes.
Somewhere between bacteria and viruses, both in size and complexity, is a third group of microorganisms, known as reckettsia, which includes the agents of diseases like typhus. Rickettsiae are classified with viruses because they use the cells of other living things in order to reproduce. The larger rickettsiae can be seen under an optical microscope, and are ten times bigger than the smallest bacteria. This hardly makes them monsters - they measure only about 0.00006 mm across, small enough to pack 9500 of them within a hair's breadth.