Cocci are round, bacilli are rod-shaped, and spirochetes look like spirals or corkscrews. Yet another way of identifying bacteria involves their reaction to a test called a Gram stain. They also must survive attacks from the body's other defenses.
Viruses also cause infection.
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.