THE empty house, in a middle-class corner of southern California, is two storeys high and boasts a three-car garage. Roses bloom around a kidney-shaped swimming pool, which is green with algae. Bill Bobbitt, a county inspector, dips a ladle into the water and brings up half a dozen wriggling larvae. Mosquitoes, and the West Nile virus that some of them carry, are thriving in California's plunging property market.

West Nile virus arrived in America in 1999 and made it to California three years later. Since then it is known to have infected 2,300 people in the state, of whom 76 have died. In Orange County this is the worst summer yet. By this point last year officials there had discovered nine birds that had been killed by West Nile virus and not one infected mosquito. So far this year they have found 219 infected birds and 75 infected mosquitoes.

Some of this rise is due to better testing and co-operation with the animal services department, which receives most reports of dying birds. But a much bigger cause is the housing crunch. Fully 63,000 homes were foreclosed in California between April and June, according to DataQuick, a property data services outfit. In the past year the number of Orange County homeowners who have defaulted on their mortgages has more than doubled. Empty houses mean untended pools. Untended pools quickly breed mosquitoes.

Dead birds are also piling up in neighbouring counties like Los Angeles, San Diego and San Bernardino, which also have high foreclosure rates. Last week 170 infected mosquitoes were discovered in the state as a whole—the highest tally ever. So far this year 13 human infections have been reported in California, but the numbers are expected to grow rapidly as the summer moves on. John Rusmisel, president-elect of the board responsible for killing the critters, says a peak in infected mosquitoes is generally followed, two or three weeks later, by a peak in human cases.

In theory, owners are supposed to keep their properties in decent shape whether they live there or not. California has even passed a bill fining banks and mortgage companies that seize properties and then allow pools to fester. But Mr Bobbitt isn't waiting for the lawyers. He has treated the pool in Santa Ana with oil and synthetic growth hormones, which will keep the mosquitoes adolescent, preventing breeding. Then he tips in a few dozen mosquito fish (Gambusia affinis), which begin happily munching larvae. You can buy a lot of the fish for what a lawyer charges per hour, and some authorities, with commendable creativity, even provide them free to help control the pests.