Ancient peoples in India, China, and Persia observed permanent immunity following recovery from smallpox and reasoned that innoculation with the dried crusts of pustules. The deadly smallpox virus, which disfigures and often causes blindness and death, was eradicated from nature early in the 1980s. A major triumph for public health, smallpox eradication remains a landmark in disease management. The last outbreak was in Somalia in 1977. There is a fear that a rogue state like North Korea may also have a stockpile of smallpox virus.
Humans thus become an accidental host for the disease when exposed to the lice.
This disease has disabled armies and devastated civilians. You might enjoy the classic reader on this disease, Zinsser's Rats, Lice, and History . Typhus fever was introduced into the Americas by the Spanish explorers where it ravished the Indian population and even now persists in endemic form in Mexico, Central, and South America. There were major outbreaks in World War I, World War II and 1951 in Korea. It is always out there, but makes an appearance when social upheaval leave people cold, filthy and hungry.