Final answer:
Malaria is a disease not caused by a bacterium; it is caused by Plasmodium parasites. Plague is caused by Yersinia pestis, epidemic typhus by Rickettsia prowazekii, and relapsing fever by Borrelia species.
Step-by-step explanation:
The disease not caused by a bacterium among the options provided is malaria. Malaria is caused by Plasmodium parasites, which are transmitted to people through the bites of infected mosquitoes. On the other hand, the plague is caused by the bacterium Yersinia pestis, epidemic typhus is caused by Rickettsia prowazekii, and relapsing fever is commonly caused by Borrelia species.
Plague, particularly the bubonic plague, has historically been a devastating disease with a significant mortality rate if not treated. It's transmitted to humans from small mammals and their fleas. Epidemic typhus is particularly associated with crowded conditions, which can be exacerbated during war and is transmitted by body lice. Relapsing fever is known for recurring episodes of fever and can be transmitted by lice or soft-bodied ticks.