The virus spread through the air, attacking the skin cells, bone marrow, spleen, and lymph nodes of its victims.

The unlucky infected developed fevers, vomiting, and rashes.

30% of infected people died during the second week of infection.

Survivors bore scars and scabs for the rest of their lives.

Smallpox had arrived.

In 1350 B.C., the first smallpox epidemics hit during the Egypt-Hittite war.

Egyptian prisoners spread smallpox to the Hittites, which killed their king and devastated his civilization.

Insidiously, smallpox made its way around the world via Egyptian merchants,

then through the Arab world with the Crusades,

and all the way to the Americas with the Spanish and Portuguese conquests.

Since then, it has killed billions of people with an estimated 300 to 500 million people killed in the 20th century alone.

But smallpox is not unbeatable.

In fact, the fall of smallpox started long before modern medicine.

It began all the way back in 1022 A.D.

According to a small book, called "The Correct Treatment of Small Pox,"

a Buddhist nun living in a famous mountain named O Mei Shan, in the southern province of Sichuan,

would grind up smallpox scabs and blow the powder into nostrils of healthy people.

She did this after noticing that those who managed to survive smallpox never got it again,

and her treatment worked.

The procedure, called variolation, slowly evolved,

and by the 1700s, doctors were taking material from sores and putting them into healthy people through four or five scratches on the arm.

This worked pretty well as inoculated people would not get reinfected, but it wasn't foolproof.

Up to 3% of people would still die after being exposed to the pus.

It wasn't until English physician Edward Jenner noticed something interesting about dairy maids that we got our modern solution.

At age 13, while Jenner was apprentice to a country surgeon and apothecary in Sodbury, near Bristol,

he heard a dairy maid say, "I shall never have smallpox, for I have had cowpox."

"I shall never have an ugly, pockmarked face."

Cowpox is a skin disease that resembles smallpox and infects cows.

Later on, as a physician, he realized that she was right.

women who got cowpox didn't develop the deadly smallpox.

Smallpox and cowpox viruses are from the same family.

But when a virus infects an unfamiliar host, in this case cowpox infecting a human, it is less virulent,

so Jenner decided to test whether the cowpox virus could be used to protect against smallpox.

In May, 1796, Jenner found a young dairy maid, Sarah Nelmes,

who had fresh cowpox lesions on her hand and arm caught from the udders of a cow named Blossom.

Using matter from her pustules, he inoculated James Phipps, the 8-year-old son of his gardener.

After a few days of fever and discomfort, the boy seemed to recover.

2 months later, Jenner inoculated the boy again, this time with matter from a fresh smallpox lesion.

No disease developed, and Jenner concluded that protection was complete.

His plan had worked.

Jenner later used the cowpox virus in several other people and challenged them repeatedly with smallpox,

proving that they were immune to the disease.

With this procedure, Jenner invented the smallpox vaccination.

Unlike variolation, which used actual smallpox virus to try to protect people,

vaccination used the far less dangerous cowpox virus.

The medical establishment, cautious then as now, deliberated at length over his findings before accepting them.

But eventually, vaccination was gradually accepted and variolation became prohibited in England in 1840.

After large vaccination campaigns throughout the 19th and 20th centuries,

the World Health Organization certified smallpox's eradication in 1979.

Jenner is forever remembered as the father of immunology,

but let's not forget the Buddhist nun, dairy maid Sarah Nelmes, and James Phipps,

all heroes in this great adventure of vaccination, who helped eradicate smallpox.