With the exception of prions, viruses are the smallest known agents of infectious disease. Most viruses are exceedingly small (about 20 - 200 nanometers in diameter) and essentially round in shape. They consist of little more than a small piece of genetic material surrounded by a thin protein coating. Some viruses also have a thin, fatty envelope surrounding their protein coat.
Viruses are different from all other infectious microorganisms because they are the only group of microorganisms that are entirely reliant on a host cell for replication. Because viruses do not eat food - instead they seize materials and energy from host cells by hijacking cellular machinery - some scientists argue that they are more like complex molecules than living creatures. Viruses are known to infect nearly every type of organism on Earth. Some viruses, called bacteriophages, even infect bacteria.
Bacteria are generally ten to 100 times larger than viruses. They are typically 1 to 3 microns in length and take the shape of a sphere or rod. Most bacteria consist of a ring of DNA surrounded by cellular machinery, all contained within a fatty membrane.
They acquire energy from the same essential sources as humans, including sugars, proteins, and fats. Some bacteria live and multiply in the environment while others are adapted to life within human or animal hosts. Some bacteria can double in number every fifteen minutes, while others take weeks or months to multiply.
Bacteria cause many types of diseases, ranging from mild skin irritation to lethal pneumonia.
Parasites are part of a large group of organisms called eukaryotes. Parasites are different from bacteria or viruses because their cells share many features with human cells including a defined nucleus.
Parasites are usually larger than bacteria, although some environmentally resistant forms are nearly as small. Some parasites only replicate within a host organism, but some can multiply freely in the environment. Parasites can be made of one cell, as in the case of Giardia, or many cells, as with parasitic worms.
In developing countries unicellular parasites, such as Plasomdium, the cause of malaria, are a major sources of disease. Waterborne parasites, such as Giardia and Cryptosporidium are the most common causes of parasitic disease in the United States.
Fungi are diverse in terms of their shape, size and means of infecting humans. Fungi are eukaryotes, meaning that like parasites, their cells have a true nucleus and complex internal structures.
They are most commonly found as environmentally resistant spores and molds, but can cause disease in humans in the form of yeasts. Fungi most often cause skin infections and pneumonia. Fungal diseases are particularly dangerous to immunocompromised people, such as those suffering from AIDS.