The history of immunology goes back to ancient Greece.  The earliest known mention of immunity was during the plague in Athens around 430 BC. Thucydides noted that people who had recovered from a previous bout of the disease could nurse the sick without contracting the illness a second time. But it would not be until 1891 that microorganisms were confirmed as the cause of infectious disease.

A progressive decline in hormone levels with age is in part responsible for a weakened immune system in aging individuals. The immune system is enhanced by sleep and rest, and is impaired by stress.