Smallpox is a very bad disease with a high mortality rate. It is caused by a virus. There are two different species of viruses that can cause the disease. They are Variola major and Variola minor. Some people also call smallpox Variola, named after the viruses' scientific name.
Only humans can get this disease. Variola major kills between 20% and 40% of those who get it. Variola minor kills only about 1%. Many people who survive become blind because of the damage the virus does to the eyes. 
During the first half of the 20th century, between 300 million and 500 million people died of this disease. Even in 1967, about 15 million people caught the disease, and about two million people died of it, according to the World Health Organization (WHO).
The first vaccine for smallpox used the results of cowpox infections. It was invented by Edward Jenner. It was used to stop people from getting smallpox. The word "vaccine" came from "vaccina", the Latin word for cow, because cowpox was used. The WHO (World Health Organisation) said that they were trying to eradicate (completely destroy) smallpox in 1963. They vaccinated people all over the world. In 1980, the WHO said the disease no longer existed, and no one would ever get sick from it again. However, live copies of smallpox are kept in different maximum-security laboratories around the world.
Some people believe that smallpox could be used as an agent for purposely infecting enemies in a war. Today, most people no longer receive a smallpox shot; but a few very, very old people still need it.
If somebody has smallpox, their entire body is covered in bumps infiltrated by blood that are worse than chicken pox.