The germ theory is a fundamental tenet of medicine that states that microorganisms, which are too small to be seen without the aid of a microscope, can invade the body and cause certain diseases.

Until the acceptance of the germ theory, diseases were often perceived as punishment for a person's evil behavior. When entire populations fell ill, the disease was often blamed on swamp vapors or foul odors from sewage. Even many educated individuals, such as the prominent seventeenth century English physician William Harvey, assumed that epidemics were caused by miasmas, poisonous vapors created by planetary movements affecting the Earth, or by disturbances within the Earth itself.

The development of the germ theory was made possible by certain laboratory tools and techniques that permitted the study of bacteria during the seventeenth and eighteenth centuries. The invention of primitive microscopes by the English...