From The Lost City of the Monkey God, this unsettling description of smallpox:

Epidemiologists generally agree that smallpox is the cruelest disease ever to afflict the human race. In the century before it was eradicated in the 1970s, it killed more than half a billion people and left millions of others horribly scarred and blind. It inflicts unbearable suffering, both physical and psychological. It usually starts like the flu, with headache, fever, and body aches; and then it breaks out as a sore throat that soon spreads into a body rash. As the disease develops over the subsequent week, the victim often experiences frightful hallucinatory dreams and is racked by a mysterious sensation of existential horror. The rash turns into spots that swell into papules, and then fluid-filled pustules that cover the entire body, including the soles of the feet. These pustules sometimes merge, and the outer layer of skin becomes detached from the body. In the most deadly variety of smallpox, the hemorrhagic form, called the bloody pox or black pox, the skin turns a deep purple or takes on a charred look, and comes off in sheets. The victim often “bleeds out,” blood pouring from every orifice in the body. It is extremely contagious. Unlike most other viruses, smallpox can survive and remain virulent for months or years outside the body in clothing, blankets, and sickrooms.

Whew. I had no idea.