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.
In the UK in 1994 it was estimated that the rat population exceeded the human population and that one in twenty homes was affected by rats. The difficulties of rat control were aggravated by the emergence of rats resistant to many rodenticides and with an instinctual ability to avoid many of the traps which were effective in the past. The rate of 150 incidents a month of surface infestation by rats in 1995 was due to a boom in the rat population in sewers. It was estimated that nine out of 10 rats seen above ground were coming from the sewers. Privatized water companies were blamed because of their tendency to cut their sewer baiting budgets by as much as 60 percent, and for baiting only when infestations were brought to their attention. Research revealed a 39 percent rise in rat infestations between 1970 and 1993.
In 1998, Vietnam faced the country's biggest rat population explosion in years. Gluttonous rodents wiped out up to 80% of the winter-spring harvest in some southern rice-growing provinces and infested more than 130,000 hectares of rice paddies nationwide.