Why are body lice associated with typhus and relapsing fever?

The body louse (Pediculus humanus humanus) is a significant vector for epidemic typhus and relapsing fever because it thrives in environments characterized by overcrowding, poor hygiene, and cold weather, which facilitate its spread between humans.123

Transmission Mechanisms

• Typhus: The body louse becomes infected with the bacterium Rickettsia prowazekii while feeding on the blood of an infected person. The bacteria multiply within the louse's gut and are excreted in its feces. Humans typically contract the disease by scratching a louse bite, which rubs the infected feces into the wound.3
• Relapsing Fever: This disease is caused by Borrelia spirochetes. Unlike typhus, neither the bite nor the excreta of the louse is infectious. Instead, human infection occurs when a louse is crushed on the skin while scratching, allowing the bacteria to enter the body.2

Because lice are sensitive to temperature, they often abandon a host who develops a high fever or dies, moving to new individuals and further spreading these pathogens.23