Which arthropod is known to transmit epidemic typhus?

Epidemic typhus is primarily transmitted by body lice, which are a type of ectoparasite that infests humans. When body lice bite an infected host, they can become infected with the bacterium Rickettsia prowazekii, which causes epidemic typhus. The lice can then transmit the bacteria to humans through fecal contamination; when an individual scratches the bite area, they can introduce the bacteria present in the lice feces into their skin. This method of transmission underlines the importance of hygiene and living conditions, as epidemic typhus outbreaks are often associated with crowded, unsanitary environments.

In contrast, other arthropods mentioned, such as ticks, mosquitoes, and sand flies, are not known vectors for epidemic typhus. While ticks are important vectors for diseases like Lyme disease and Rocky Mountain spotted fever, mosquitoes are primarily known for transmitting viral infections such as malaria and dengue fever. Sand flies are associated with the transmission of leishmaniasis. Thus, body lice's specific role in the transmission of epidemic typhus is what makes it the correct answer in this context.