Alastrim

Alastrim, also known as variola minor, is the milder strain of the variola virus that causes smallpox.

Variola minor is of the genus orthopoxvirus, which are DNA viruses that replicate in the cytoplasm of the affected cell, rather than in its nucleus. Like variola major, alastrim is spread through inhalation of the virus in the air, which can occur through face-to-face contact or through fomites. Contagion with variola minor confers immunity against its more dangerous form, variola major.

Variola minor is a less common form of the virus, and much less deadly. Although alastrim has the same incubation period and pathogenetic stages as smallpox, alastrim is believed to have a mortality rate of less than 1%, as compared to smallpox's 30%.

Because alastrim is a less debilitating disease than smallpox, patients are more frequently ambulant and thus able to infect others more rapidly. As such, variola minor swept through the USA, Great Britain, and South Africa in the early 20th century, becoming the dominant form of the disease in those areas and thus rapidly decreasing mortality rates.