The major reason why the average human lifespans were less than 50 years until fairly recently is due to infectious diseases like tuberculosis, cholera and smallpox. It was also because the causes and cure of other major diseases like malaria and plague had not yet been found.
The lifespan of an entire population of people is known as the life expectancy, while the lifespan of an individual is a measure of the length of a person's life. Until recent years, causes, modes of transmission and treatment of common diseases were not known. Access to good medical care was difficult and hygienic conditions then were very poor. For example, it wasn't until the mid-1800's that people came to know that washing of hands could stop the spread of diseases, and until the late 1800's doctors didn't adequately wash their hands before performing a surgery.
Unhygienic conditions also made it easier for deadly infectious diseases like the Bubonic plague, the Black plague and the Spanish flu to spread, further reducing the already low life expectancy.
Today, thanks to good hygiene, better understanding of transmission of microbes, environmental sanitations, early immunizations and access to clean water, life expectancy has increased.
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