These first microbe hunters proposed the revolutionary idea that behind every infectious disease lurked a specific type of microbe. By applying his theory, Pasteur managed to combat the spread of diseases such as tuberculosis through "pasteurizing" milk and Robert Koch established his "postulates"–criteria to link a specific microbe to a specific disease–that scientists use to this day.

Evolution

Natural selection determines the fate of all living things. If a species can "adapt" to its environment, it will be more likely to reproduce more successfully than less well-adapted organisms. In contrast to humans, microbes can multiply and mutate in a matter of minutes. Through the use of antibiotics, vaccinations, and even simpler preventive measures, however, humans can tip the scales in their favor–at least in the short term. Once a strain of bacteria mutates and acquires "antibiotic resistance," the microbes have the advantage again.

Ecology

To analyze the connection between humans and microbes, we also need to consider ecology and ecosystems. An ecosystem is a bounded system where living organisms co-exist in a constant state of interaction and interdependency. The human body is one such ecosystem. For example, billions of benevolent bacteria thrive in the human digestive tract while preventing harmful bacteria to settle there.To understand the delicate balance between humans and microbes, imagine a seesaw. On one end sit microbes, with their biological advantages, and on the other end are humans, with their adaptations. In a stable ecosystem, the seesaw balances evenly, with occasional fluctuations. When an ecosystem is suddenly disrupted, the seesaw tilts and things are thrown out of balance. During an epidemic, regaining the balance takes time and knowledge of what caused the disruption. By understanding the complex relationship between habit, human, pathogen, and vector behavior, and other ecological factors, we can fight infectious disease more effectively.

Culture

Different cultures have different views of infectious disease; these views determine how infectious disease is defined, diagnosed, and treated. For example, in some parts of China, people consider measles an important rite of passage to adulthood instead of a threatening disease. In addition, some cultures have developed effective methods for treating disease that are not based on the Western biomedical perspective and, therefore, could be construed as unscientific and useless.

In fact, these cultural methods can offer insight into the treatment of infectious disease. To understand how evolution, ecology, and culture fit into the larger picture, let us look at the different ways humans are exposed to microbes. After a discussion of the factors that lead to infection, we'll examine how outbreaks occur.

Finally, we'll explain how microbes travel throughout populations during epidemics, and globally in pandemics. Then, we'll consider how humans can prevent the spread of infectious disease by taking action.