Rats spread diseases such as Lymphocytic choriomeningitis, or LCMV, and leptospirosis through their saliva and urine, according to Orkin. Rats spread the bubonic plague, also known as the Black Death, one of the most famous and historically significant diseases, which was spread through fleas that bit both rats and humans. Rats also spread salmonellosis and rat-bite fever.
Transmission of LCMV, leptospirosis, salmonellosis and rat-bite fever usually occurs directly through a bite or scratch, or by coming into contact with urine or scat, reports Orkin. However, diseases such as the plague are spread indirectly through fleas and other parasites that travel from the rat to another host. For example, Colorado Tick Fever and Cutaneous Leishmaniasis are spread by ticks and sand flies respectively that transfer from a woodrat.
Not all species of rats are carriers for certain diseases, according to Orkin. The cotton rat and the rice rat spread hantavirus, a viral disease, but the Norway rat and the roof rat do not. In the case of leptospirosis, even indirect contact through drinking or swimming in infected water can spread the disease.
In addition to disease, rats also can be a source of allergens, warns Orkin. Their droppings, dander and hair all can trigger allergic reactions in people.