They cannot live on their own, but instead can only grow in the cells of other living things. A virus enters a cell, takes it over, kills it, and moves on to the next cell. This continuous destruction explains why some viruses cause so much damage. Rickettsia live within hosts such as ticks, which pass the microorganisms on to humans. Chlamydia are a large group of bacteria-like organisms that have become an alarmingly common cause of sexually transmitted disease.
Fungi are another group of pathogenic, or infection-causing, microorganisms.
Fungi such as yeasts and molds can enter the body and wreak havoc, especially in those with weakened immune systems. There are about 100,000 known fungi, but relatively few cause human disease. Infectious diseases are transmitted in several ways. Most often, we experience infection after direct contact with the infectious agent. For example, if a person with a cold coughs into his or her hand and then picks up a public telephone, the people who use that phone shortly afterward will likely pick up the microorganism.