Nature
The population of rats increases rapidly where there are problems of public health and sanitation, notably where there is inadequate collection of refuse or where disposal of refuse is casual. The presence of rats among humans raises a number of serious health risks. One of the most dangerous diseases communicable from rats is Weil's disease which can cause jaundice, renal failure, even death. Rats can also carry salmonella, listeria, and Lyme disease. Other than their major role in spreading disease (including plague), rats can cause severe losses to stored grains and other foodstuffs, especially in developing countries.