How did war, famine, and overcrowding contribute to outbreaks of louse-borne disease?

War, famine, and overcrowding create ideal environments for the transmission of louse-borne diseases, such as epidemic typhus and trench fever, by fostering conditions where the body louse thrives.123

These disasters often force people into crowded, unhygienic living situations—such as trenches, prisons, refugee camps, or famine-stricken areas—where individuals are in close proximity, facilitating the rapid spread of lice from person to person. The human louse feeds on human blood and transmits pathogens like Rickettsia prowazekii or Bartonella quintana through its feces. Infection typically occurs when a person scratches a louse bite, inadvertently rubbing the contaminated feces into the wound or mucous membranes.123

Historically, these outbreaks were frequent by-products of such crises because the lack of sanitation and inability to maintain personal hygiene allowed louse populations to flourish. The association was so strong that epidemic typhus was historically referred to as "war fever" or "jail fever," reflecting the environments where it most commonly devastated populations.12