Bacteria

The most abundant organisms on Earth, bacteria live almost everywhere: in the soil and water, in plants and animals. Whether they take the form of spheres, rods or spirals, bacteria consist of a single cell. Unlike the cells of animals and plants, bacterial cells lack a nucleus, but they can carry out all necessary life functions. Most bacteria are parasites, although a few manufacture their own food. Some of these parasites are very helpful -- they aid in many bodily functions including digestion, and help with other processes, such as decomposition of soil and changing of milk into cheese. Disease results, however, when bacteria multiply rapidly (each cell simply divides into two identical cells) and damage or kill human tissue, as in pneumonia and tuberculosis. Diseases can also produce toxins that damage or kill human tissue, as in food poisoning or cholera. Sometimes bacteria in the body are helpful for a while, and then something in the body or the bacteria changes, causing destruction in the host.

Viruses

By far the smallest microbes, viruses can appear as spirals, 20-sided figures or even more complicated forms. They consist mainly of genetic material--DNA or RNA. They are not cells, however, and cannot carry out life functions on their own. Living inside the cells of other species, viruses use the host cells to grow and produce new viral particles. As they take over genetic material to reproduce themselves, the host cells often die. Found in all groups of living things, from bacteria and fungi to plants and animals, hundreds of the known viruses can cause many kinds of infections, chickenpox, measles, flu, colds, polio, and AIDS. Viruses cannot move by themselves and must be carried to cells by air currents and then by body fluids to the cells. Some viruses may lay dormant for years before becoming active, as with AIDS. Most diseases come from other species, for example: smallpox from dogs or cattle, hemorrhagic fevers from rodents and monkeys, tuberculosis from cattle and birds, common cold from horses, and AIDS from African monkeys.

Fungi

Fungi include yeasts (one-celled), and mushrooms and molds (multi-celled). Unlike plants, fungi do not make their own food. Some species of fungi get their nutrition by breaking down remains of dead plants or animals. Others are parasites. Examples of fungal infections include athlete's foot and ringworm.

Nematodes

Other microorganisms break down body tissues or absorb digested food. They can cause anything from skin infections to internal disorders that can lead to death. The group called helminths includes flukes, roundworms, and tapeworms; these are many-celled animals with developed organs. Among the numerous types, some are parasites--organisms that live in or on another species, usually harming the host species in the process. Because of their size, parasitic worms grow outside of cells and can reach an astronomical size of 30 feet in length.

Protozoa

Protozoa consist of a single cell that includes a nucleus. The cell also contains structures that carry out specific processes needed for life functions. A diverse and complex group, protozoa range through many shapes and sizes. They can be parasitic, needing to live within another organism, or free-living in moist habitats. The similarity of inner structures of protozoan and human cells makes it difficult to treat infections caused by protozoa. Drugs that may destroy the protozoan may also destroy human cells. Protozoan infections include amebic dysentery, malaria, and African sleeping sickness.