Alastrim, also known as 'variola minor'* was a mild strain of the Variola virus; 'variola major' is better known as smallpox. Alastrim came to the attention of the medical establishment c. 1896-1910. Identification was difficult, as it shared nearly every trait with smallpox, but was so mild that some thought it might be more closely related to chicken pox. It produced a headache, myalgias, and rash like smallpox, but slightly less severe fever and pox. However, the big difference was a mortality rate of just 1% (smallpox was about 30%). Alastrim saved many lives, as an infection with alastrim provided immunity from smallpox.
Alastrim spread faster than smallpox -- perhaps because lesser symptoms meant more people were ambulatory and spreading it for longer. It became the dominate strain in the United States and in parts of South America, Europe, and Africa. Unfortunately, once the smallpox vaccine became available, public confusion between smallpox and alastrim would lead many to think that the vaccine was unnecessary -- after all, practically everyone had survived the last outbreak, right? Alastrim went extinct a few years before smallpox, in 1977, as it was likewise prevented by the smallpox vaccine.
*Because alastrim tended to spread quickly in bursts around the world, it collected many labels; it was a.k.a. kaffir pox, Cuban itch, West Indian pox, Sanaga pox, and the Australian Disease (in New South Wales). Additionally, like smallpox, the pox marks went through a stage of milky white vesicles, leading to another set of names: white pox, cottonpox, and milk pox. The South African name, amaas, was commonly used in medical circles in the early 1920s, as was the scientific-sounding 'pseudovariola'. Alastrim, meanwhile, comes from the Brazilian name, derived form the Portuguese alastrar, meaning 'spread', purportedly an allusion to fire spreading.
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