A communicable disease is any disease that can be transmitted from one organism to another. Agents that cause communicable diseases, called pathogens, are easily spread by direct or indirect contact. These pathogens include viruses, bacteria, fungi, and parasites. Some pathogens make toxins that harm the body's organs. Others actually destroy cells. Some can impair the body's natural immune system, and opportunistic organisms set up secondary infections that cause serious illness or death. Once the pathogens have multiplied inside the body, signs of illness may or may not appear. The human body is adept at destroying most pathogens, but the pathogens may still multiply and spread.

The pathogens responsible for some communicable diseases have been known since the mid-1800s, although they have existed for a much longer period of time. European explorers brought highly contagious diseases such as smallpox, measles, typhus and scarlet fever to the...