A photomicrograph of the spores that cause histoplasmosis.
Credit: Dr. Libero Ajello/CDC
Other Microbes
In addition to viruses and bacteria, three other major types of microbes—and one newly discovered type—can cause infectious disease. As with viruses and bacteria, not all of the species in each of these categories are infectious to humans. But many of the world’s most prevalent infectious diseases are caused by microbes included in the groupings below.
Fungi: These organisms cause a wide variety of diseases in humans, ranging from athlete’s foot to ringworm to deadly histoplasmosis. Some fungi, such as yeasts, are comprised of a single cell, but most are multicellular. They are found in the air, in soil, on plants, and in water. Only about half of all known fungi are harmful. Many perform vital functions such as helping materials decay and decompose in the environment. They reproduce primarily by forming spores that float in the air. These spores can land on human skin or be inhaled, which is why most fungal infections start on the skin or in the lungs. A weakened immune system can make people more prone to fungal infection. So can taking antibiotics, which reduce the bacteria in the body that keep some fungal communities (such as yeast) from growing unchecked.
Prions evoke no immune response and resist heat, ultraviolet light, radiation, and sterilization, making them difficult to control.
Protozoa: Amoebas and paramecia may be the most familiar examples of these single-celled microbes. Able to move rapidly and flexibly because they do not have cell walls, the different species that fall under this category have little in common otherwise. Protozoa typically enter human hosts through contaminated water or food or by the bite of an infected arthropod, such as a mosquito. They are able to multiply in humans, so the presence of just one protozoan can lead to serious infection. These parasites cause some of the deadliest infectious diseases worldwide, including malaria and amebic dysentery.
Helminths: Parasitic worms, or helminths, cause mild diseases such as swimmer’s itch but also more serious illnesses such as schistosomiasis, a disease spread to humans via snails. Tapeworms, flukes, and roundworms comprise the three main categories of helminths. Unlike lice and fleas, which are external parasites, helminths live inside a host. Their presence typically disrupts the host’s nutrient absorption, causing weakness and a greater vulnerability to disease. Helminth eggs can contaminate food, water, soil, feces, air, and surfaces such as doorknobs and toilet seats. The eggs enter the human body through the mouth, anus, or nose and often hatch, grow, and multiply in the human intestine, though they may infect other areas of the body. Some helminths are large enough to be seen with the naked eye and can live within their hosts for years. Proper sanitation and thorough cooking of meat can help prevent the transmission of helminths.
Prions: A newly recognized class of infectious agents, the prions or proteinaceous infectious particles, consist only of protein. Prions are thought to cause variant Creutzfeldt-Jakob disease in humans and bovine spongiform encephalopathy (mad cow disease) in cattle. These proteins are abnormally folded and, when they come in contact with similar normal proteins, turn them into prions like themselves, setting off a chain reaction that eventually riddles the brain with holes. Prions evoke no immune response and resist heat, ultraviolet light, radiation, and sterilization, making them difficult to control.
