Infections caused by microorganisms are often spread more easily in an unsanitary environment. Even today, for example, Escherichia coli infections of food and between people are still commonly caused by poor hygienic practices, such as the failure to properly wash hands after toileting.

Because of the association between microbial infection and sanitary conditions, many public health initiatives and regulations have been instituted around the world. Before the recognition that microbes caused infections, and even before the realization that microbes existed, public health was a foreign concept. The history of public health parallels advances in the understanding of microorganisms and disease.

Prior to the fourteenth century, public health was nonexistent. The sanitary environment in urban centers was appalling by today's standards. However, at that time there was no knowledge that, for example, the flow of raw sewage alongside streets was connected to illness...