The immune system contains many biological structures and processes within the body that protect it against disease. To function properly, your immune system must detect a wide variety of agents, known as pathogens, from viruses to parasitic worms, and distinguish them from the organism's own healthy tissue.
You may wonder why you are sick more or less often than your partner, co-workers and neighbors. You may wonder why one person hacking on the airplane successfully sickens the passenger to his right but not the one to his left. The answer depends on the health of your immune system!