The two classic varieties of smallpox are variola major and variola minor. Smallpox is caused by either of the two viruses, Variola major and Variola minor. This form of variola major was more easily confused with chickenpox. Infection with either variola major or variola minor confers immunity against the other. Infection with variola minor conferred immunity against the more dangerous variola major. Infection with "v. minor" induces immunity against the more deadly "variola major" form. The last naturally occurring case of "Variola major" was diagnosed in October 1975 in Bangladesh.