A baby with smallpox (from the CDC)
Smallpox was a very serious disease caused by a virus. Many people died of it. Smallpox caused little bumps on your skin, like chickenpox but much more serious. About two to five of every ten people who got smallpox died of it. There was (and is still) no treatment that worked.
Smallpox is probably originally a Central Asian disease. The first time we know of anybody having smallpox (at least we think it was smallpox) is in the Parthian empire about 150 AD. It may have been a smallpox epidemic that weakened the Parthian army so that the Roman army was able to push it back (or it may have been measles). But when the Roman soldiers returned home from that war, about 160 AD, they brought the smallpox with them. Many soldiers, and many other people in the Roman Empire, died of smallpox over the next hundred years. Smallpox spread the other direction, east to China, about the same time, possibly weakening and ending the Han Dynasty and leading to civil war.
The next smallpox outbreak we know about was in India,
around 400 AD. Doctors in India
described it as like grains of rice all over your skin, with burning pain.
They blamed it on a new goddess,
There was a serious outbreak of smallpox in Europe about 581 AD. After that, there were smaller outbreaks in Europe here and there, but it's just like chickenpox, once you get it once you usually can't get it again, and so once a lot of people were immune future outbreaks were not as serious.
About 900 AD, the Islamic doctor Al Razi was the first to show that measles and smallpox were two different diseases. By about 1000 AD, doctors in India and China were beginning to figure out how to prevent smallpox, though they still couldn't help you once you had caught it. In India, doctors rubbed pus from a sore on a person with smallpox into a small cut on your arm. In China, doctors blew powdered smallpox scabs up your nose. This worked like a vaccination. These ways of vaccinating people became common in China and India and in the Islamic world, and also in East Africa, but nobody in Europe even knew about it.