Brill–Zinsser disease is a delayed relapse of epidemic typhus, caused by Rickettsia prowazekii

Rickettsia prowazekii

Rickettsia prowazekii is a species of gram negative, Alpha Proteobacteria, obligate intracellular parasitic, aerobic bacteria that is the etiologic agent of epidemic typhus, transmitted in the feces of lice. In North America, the main reservoir for R. prowazekii is the flying squirrel. R...

. After a patient contracts epidemic typhus from the bite of an infected louse

Louse

Lice is the common name for over 3,000 species of wingless insects of the order Phthiraptera; three of which are classified as human disease agents...

 (Pediculus humanus

Pediculus humanus

Pediculus humanus is a species of lice that infects humans. It comprises two subspecies:*Pediculus humanus humanus Linnaeus, 1758 – the body louse*Pediculus humanus capitis De Geer, 1767 – the head louse...

), the rickettsia

Rickettsia

Rickettsia is a genus of non-motile, Gram-negative, non-sporeforming, highly pleomorphic bacteria that can present as cocci , rods or thread-like . Being obligate intracellular parasites, the Rickettsia survival depends on entry, growth, and replication within the cytoplasm of eukaryotic host cells...

 can remain latent and reactivate months or years later, with symptoms similar to or even identical to the original attack of typhus, including a maculopapular rash

Rash

A rash is a change of the skin which affects its color, appearance or texture. A rash may be localized in one part of the body, or affect all the skin. Rashes may cause the skin to change color, itch, become warm, bumpy, chapped, dry, cracked or blistered, swell and may be painful. The causes, and...

. This reactivation event can be transmitted by louse bites, and form the focus for a new epidemic

Epidemic

In epidemiology, an epidemic , occurs when new cases of a certain disease, in a given human population, and during a given period, substantially exceed what is expected based on recent experience...

of typhus.