Smallpox. Man with smallpox lying in bed. He has pus-filled blisters covering his face, hands and torso due to smallpox, a highly-infectious viral disease that is unique to humans. It is caused by two virus variants called Variola major and Variola minor. V. major is the more deadly form, killing between 20-40% of those infected. Survivors are often left blind, and all have scarring. Smallpox was common up until the 20th century, when the World Health Organisation's (WHO) global vaccination campaign totally eradicated smallpox in the 1980s.