The ancients thought that epidemics and diseases were divine punishment of the people for their sins. However, by the time of Moses, the Egyptians and Hebrews had come to believe that leprosy could be transmitted by contact with lepers. In Europe, around 430 B.C. Thucydides had concluded that some plagues were contagious. By the Middle Ages, many believed this, and they fled cities to escape the diseases. Fracastorius, in 1546, first proposed a theory that communicable diseases were caused by living agents, passed from one person or animal to another. However, he had no way to test this theory.

With the discovery of microorganisms in the late 1600's by Leeuwenhoek, people began to suspect that the microorganisms might cause disease, but the techniques of the times were not able to prove this. It was not until 1876 that Robert Koch offered convincing proof of the "germ theory" of disease when he showed that Bacillus anthracis is the cause of anthrax, a serious and often fatal disease of humans, sheep, and other animals. With his microscope, he observed B. anthracis bacteria in the blood and spleen of dead sheep. He then inoculated mice with the infected sheep blood and was able to recover B. anthracis from the blood of the mice. In addition, he grew the bacteria in pure culture and showed that they caused anthrax when injected into healthy mice. From these experiments and later work with Mycobacterium tuberculosis Koch formalized a group of criteria for establishing the cause of an infectious disease, known as Koch's Postulates.

After learning some of the many ways in which human beings can protect themselves against organisms and other foreign materials, it may seem surprising that microorganisms are actually able to infect people and cause disease. However, microbes and viruses have a vast multitude of ploys for gaining entry into people, living in or on the human body, and countering body defenses. In this chapter, we will explore some of the ways in which microorganisms and viruses colonize the human host and either live in harmony with the host in the normal flora or cause disease. We will also see how the host responds to some of the disease-producing strategies of microbes and how microbes sometimes evade host responses. The study of host-microbe interactions has become very exciting, as techniques developed for genetics, molecular biology, and immunology have been used to determine how human and animal hosts interact with microbes and how microorganisms cause disease.