Popular worldwide until the late 19th century, miasma theory claims that disease is spread by "miasma" or bad, polluted air. The miasma itself was thought to be caused by rotting organic materials such as food, feces, and corpses. Early solutions involved urban sanitation to prevent build-up of rotten goods.

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In Philadelphia, fear of miasma likely contributed to William Penn's insistence on wide blocks with green spaces and later leaders' creation of an underground sewer system.

In the 18th century, as ideas and new medical knowledge continued to spread during colonial 

wars and trade, miasmal theory was being challenged.

A minority of medical professionals began thinking that it was not bad air that caused illness, but contact between people. The process of coming into contact with an infected person and becoming ill was known as "contagion." 

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Part of miasma theory is the belief that air affects bodily fluids. These fluids, as proposed by Hippocrates, the father of Western medicine, are

• black bile

• yellow bile

• phlegm

• blood.

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Bile, phlegm, and blood are all bodily fluids, but they do not act the way humorists believed. This theory also fell out of favor in the 19th century, though it had influenced medicine for centuries​

As contagion theory was being proposed, other scholars suggested it was not bad air or contact themselves which caused disease to spread. Rather, there were small particles carried by the air or present on skin and in bodily fluids which were passed to people. These particles, known as germs, infected a person with disease.

Germ theory is now understood to be the correct theory of disease and is the basis for modern medicine. Although the beginnings of germ theory were present as the Philadelphia Lazaretto's was built and operating, it was not accepted on a large scale until the end of the hospital's use.