MICROBES
Although microbes have existed for millions, and even billions of years, their presence was not detected until the seventeenth century. In 1683 Dutch merchant Antony van Leeuwenhoek, who made microscopes as a hobby, detected "wee animalcules" in scrapings from his teeth. Another 200 years would pass, however, before scientists would establish the relationship between microbes and disease.
Although some microbes are deadly, most are harmless, and some are extremely beneficial. These microscopic organisms can be found virtually anywhere- in air, water, plants, animals, and humans.
Grouped by physical and behavioral characteristics, microbes fall into the following major categories:
*Viruses (singular: virus) are the smallest and simplest microbes, just a ball of genes wrapped in a shell - about a millionth of an inch across. No one knows how long they've been on Earth or how they evolved. They reproduce by injecting their genes into a cell to produce thousands of new viruses.
*Bacteria (singular: virus) are the much larger than viruses - about 125,000th of an inch long. If a virus were human-sized, a bacterium would be about as big as the Statue of Liberty. These on-celled bodies either make their own food through chemical processes or feed on live hosts or dead matter. Bacteria have existed on Earth for more than 3.5 billion years.
*Protozoa (singular: protozoan) are a group of one-celled predators and parasites similar to bacteria but about 1,000 times larger. Examples include amoebae and paramecia. Protozoa made their debut on Earth more than 1.8 billion years ago.
*Fungi (singular: fungus) In nature, fungi are decomposers. They break down matter into nutrients and minerals that plants and animals reuse. Of the 100,000 known species of fungus, familiar examples include mushrooms, yeast, mold and mildew.