Disease-causing microbes attack by invading the body and multiplying. To do so they must penetrate the body's first line of defense: protective barriers like dry, unbroken skin and tears, which continually wash the eyes. If the microbes succeed, infection occurs. Then the body's second line of defense—the immune system—goes to work to destroy the invaders or render them harmless.
Human behavior also joins the battle. To prevent or overcome infection, people use native plants with medicinal power, avoid polluted water, and take other preventative measures. And scientists develop new medicines and vaccines to defeat the disease-causing microbes that have been identified.
Despite all efforts, the microbes often win, and the infections they cause become diseases. These microbes can then leave the body the same way they entered it—through the nose, skin, and other areas—and spread
