Numerous potentially fatal diseases are transmitted to humans by rats and mice. The transmission is usually indirect. What people need to fear most is the rodents’ urine, fecal droppings or saliva, most of which can be overlooked by the human eye unless a person is on a serious rat- or mouse-hunt.

Some of the known diseases carried by rats include bubonic plague, dysentery, spotted fever, rabies, a type of meningitis and a respiratory disease called hantavirus.

If a mouse is left to roam around food, the amount of food the vermin contaminates will be 10 times the amount it actually eats, according to the pest-control company Orkin Inc.

Since 1993, when hantavirus was first detected in the United States, about 380 people have been infected with the disease; 136 died, reports the U.S. Centers for Disease Control and Prevention. Although no human cases have yet been found in Indiana, 30 states, from the west and southwest to the East Coast have reported human cases of hantavirus pulmonary syndrome.

The disease starts out with flulike symptoms, often including abdominal pain, vomiting and diarrhea. After the initial phase of the illness, severe coughing and shortness of breath occur.

Hantavirus is transmitted by infected mice through their urine, droppings or saliva. Humans can become infected when they breathe in aerosolized virus or when they touch something that has been contaminated with mice urine, droppings or saliva, then touch their nose or mouth.

Researchers also suspect people can become sick from eating food contaminated by urine, droppings or saliva from an infected mouse, according to the CDC.

Another disease, lymphocytic choriomengitis, is spread by the common house mouse. An infected mouse sheds the virus in its urine, and chronically infected female mice transmit the infection to their offspring. As with the hantavirus, it is spread when a person inhales aerosolized particles of infected rodent urine, feces or saliva. Direct exposure can occur through a small cut or other open wound in a person’s finger or hand.

Although lymphocytic choriomengitis is usually not fatal, it causes high fever, malaise, head and muscle aches, and other flu-like symptoms. Deafness because of nerve damage and arthritis can be long-term effects. Infection of the human fetus during early stages of development can lead to developmental problems or even fetal death.

Tularemia can be carried to humans when they eat or drink food and water contaminated by infected rodents. It also is spread to humans by a tick or other insect after the insect has first bitten an infected rodent. The bacteria also can be inhaled, causing fever, chills, cough and progressive respiratory problems. While tularemia is sometimes called “rabbit fever,” it also infects mice and rats and has been found in all U.S. states except Hawaii.

But a more overlooked health consequence of rodent infestation is allergies. In one study of eight urban communities by researchers at Johns Hopkins University in Baltimore, 95 percent of all homes had mouse allergen in at least one room.

Skin tests on asthmatic U.S. children in the largest cities found 18 percent had sensitivity to mice and 20 percent had sensitivity to rats, indicating they had been exposed to allergens such as mice and rat dander, urine, droppings and saliva.