One virus was the scourge of humans

As bad as Covid-19 has been, it is not even close to the worst viral disease that has swept humanity.
That honor probably goes to smallpox, a disease so toxic that it wiped out entire populations, killing up to 500 million people in the 20th century alone. It was especially deadly for children, killing up to 80 percent. Survivors of any age were left disfigured, blind, or both. After exposure, symptoms began within a week to 19 days. High fever, fatigue, aches, and vomiting appeared first, followed by red sores on the face, hands, arms, and, finally, trunk of the body. These sores left deep, pitted scars on survivors.
According to estimates by the Centers for Disease Control, the Covid virus kills between 0.26 percent and 0.4 percent of infected people. Smallpox killed no less than 20 percent and up to 60 percent in some populations.
According to the Annals of Internal Medicine, the earliest written accounts were from China in about 400 BC, but possibly earlier.

The good news
Today smallpox is gone. The last case in the U.S. was in 1949 and the last case in the world was in 1978. Today the only remnants of smallpox are the light scars left by vaccinations on people born before the 1980s. In 1979, it was declared eradicated after massive inoculation campaigns on every continent. It is thought to live only as a sample in three labs in the world.

First vaccinations
For more than a thousand years, people knew that once a person contracted smallpox, they would ever after be immune. This knowledge led to the first genuine vaccinations.
In China, as early as 400 BC, smallpox scabs were ground up and injected into the noses of healthy people.
The first western experimentation was in 1789 by English doctor Edward Jenner, who found that a similar virus, cowpox, could protect humans. The technique, which used fluid from an active smallpox sore, was scratched into the skin or vein.
The technique was not perfect. People contracted a fever and perhaps some sores but recovered. However, there was a risk of contracting the active disease.