These super-rats, immune to traditional poisons and capable of eating their way through sewer pipes, are less afraid of humans than their predecessors. Their increase in number, to plague proportions in some cities, is believed to be caused by the recent milder winters and the rising number of fast food outlets - generating more food waste in the streets - making it easier for rats to survive. The super-rats are believed to carry more disease than previously thought and pose a serious health risk. Evidence exists that these super-rats are developing degrees of immunity to existing rodenticides while others are developing behaviour patterns, testing small amounts of bait foods before consuming sufficient quantities for poisons to work.