Rickettsia prowazekii is a species of gram negative, ro shaped, aerobic bacteria that is the etiologic agent of epidemic typhus, transmitted in the feces of lice and fleas. R. prowazekii is often surrounded by a protein microcapsular layer and slime layer; the natural life cycle of the bacteria generally involves a vertebrate and an invertebrate host, usually an arthropod, typically a louse. A form of R. prowazekii that exists in the feces of arthropods remains stably infective for months. Henrique da Rocha Lima , a brazilian doctor discovered this bacteria, in 1916. Vaccines against R. prowazekii were developed in the 1940s, and were highly effective in reducing typhus deaths among U.S. soldiers during World War II. Immunity following recovery from infection with, or by immunization against, R. prowazekii is lifelong in most cases. However, R. prowazekii can establish a latent infection, which can reactivate after years or decades (referred to as Brill-Zinsser disease). R. prowazekii also appears to be the closest free-living relative of mitochondria, based on genome sequencing.