Infection occurs when microorganisms like bacteria, viruses, fungi and parasites invade and grow inside our tissues. These germs can damage cells and even kill us. Our bodies defend themselves by releasing chemicals that destroy the invaders, and by triggering an immune response. But some germs are smarter than others, and they’ve evolved ways to evade or suppress our defenses.
Bacteria cause many infections, including strep throat, tuberculosis and staph skin infections. Viruses cause colds, the flu and some sexually transmitted diseases like genital herpes and chlamydia. Fungi cause infections such as athlete’s foot and ringworm. Parasites include tapeworms (like those that cause trichinosis and hookworm), malaria protozoa and helminths, which live in the intestines, liver, lungs and brain.
These pathogens can get into the body through mouth, eyes or genital openings, through wounds that break the skin, and through bugs, ticks and fleas that carry them into the body (the “vehicle” of transmission). Some infections, such as those from head lice and some sexually transmitted infections (STIs), are spread directly between people. Others are spread through the environment, such as from contaminated toilets, sinks, benches and desks, or from shared play items, food or containers of liquid.
The presence of germs triggers a inflammatory response that recruits immune system macrophages, which surround and destroy the pathogens. But the inflammatory response can be toxic to our own cells, and in those with compromised immune systems (as in HIV infection, cancer patients receiving chemotherapy or organ transplant recipients) can lead to serious illness and death.