7.7: What causes infectious diseases?
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Infectious diseases are transmitted between individuals by infectious agents, known as pathogens [path-oh-jens], from the Greek word pathos (to suffer) and genès (to produce). Pathogens produce a lot of human suffering and disability across the world, including in relatively wealthy nations like the United States. Most people have heard of at least some types of pathogen, for example bacteria or viruses.
The wider causes of infectious diseases range from insanitary living conditions in impoverished communities, to inadequate hygiene in the high-tech environments of modern hospitals (Figure \(\PageIndex{1}\) ). The impact of infectious diseases is therefore unequally distributed around the world, not only between countries, but also between individuals and groups within the same population.
Figure \(\PageIndex{1}\). Infectious diseases are a threat everywhere: (a) A shanty settlement without piped water or sanitation in Indonesia. (b) Testing for pathogens in a hospital laboratory in England.
Human biology is another factor to consider in explaining the cause of infectious diseases. Infancy and old age, inadequate nourishment, other illnesses and some types of medication can all create conditions in the body in which infection is more easily established. In addition, there are individual human behaviors, habits and traditional practices that contribute to the causes of infectious diseases by spreading pathogens from one person to another. These behaviors give some clues about the routes by which pathogens can be transmitted, as the next section describes.