1950 in which 16 volunteers still developed the infection after 11–25 months of primary infection. It is now known that the longevity of immunity depends on the strains of the bacterium. When reinfection occurs with the same strain as the previous infection, there can be immunity for 5–6 years in monkeys. But in humans, immunity declines after one year, and disappears within two years. References Rickettsiaceae Bacteria described in 1995 Rickettsioses