SAN DIEGO -- At one Mira Mesa home the notices on the door are collecting, right along with the algae in the backyard swimming pool.

Steve Eisele with San Diego County's Vector Control Program is seeing it more often.

Eisele said, "Used to be maybe we'd get one or two in a month but since the housing boom, or collapse rather, probably 5 or 6 a week."

The home sits vacant and its status is unclear, but experts said what is clear is that left untreated the swimming pool could be a dangerous mosquito breeding ground. It is something neighbors said is a concern.

"I knew there was a pool but never thought the idea that it would be a threat of mosquitoes and all that, which if true it will be," said neighbor Ilario Fabbrini.

Health officials said a tough economy combined with unprecedented foreclosures seems to be leading to more unkept pools that could pose a public health problem.

Experts said different types of mosquitoes that are found in green pools are very capable vectors of West Nile virus.

A helicopter's bird's eye view revealed problems county health officials would never spot from the ground.

Vector control technician Steve Rivera spends a lot of time analyzing dozens of pictures he takes from the air.

"You can see growth in the pool; some algae and things," said Rivera.

When vector control technicians find a foreclosed home with a problem, they are likely to send in mosquito fish -- a very important tool in the fight against West Nile.

"Without fish technicians we would have to make return trips and check and treat repeatedly, whereas with the fish they can keep it under control for long term," said Rivera.

Last year, the county had 35 West Nile cases, and technicians hope to keep the numbers down but said it is shaping up to be a busy season.

Vector teams found and treated nearly 1,000 green pools last year from April through late July.

This year they've already found more than 900 after just two months.