Did you know there are 23 types of mosquitoes in the East Bay, and a female can lay 400 eggs in two tablespoons of standing water? Did you know rats and mice are known carriers of bacterial disease? That a skunk bite can lead to rabies?

These are among the concerns of the Contra Costa Mosquito & Vector Control District, which services the county's 735 square miles but remains one of its least understood public health agencies.

The district was formed more than 86 years ago out of necessity, spokeswoman Deborah Bass said: "In 1927, the marsh mosquitoes were so bad that schools closed. There were reports of children's arms being blackened with mosquitoes."

The miserable little bloodsuckers remain the district's biggest concern, occupying 12 of its 17 technicians and most of its attention, but its duties go far beyond pesticides and smogging.

The upper floor of the Concord-based facility is dedicated to lab work, where live specimens are nurtured, tested and examined. The insectary houses pans of mosquito larvae at various stages of growth, and a screened box is filled with nasty-looking adults.

"They're our control group," said scientific program manager Steve Schutz. "One of our concerns is mosquitoes developing a tolerance for some pesticides. By comparing how long it takes to kill these and the wild ones, we can tell if the wild ones are developing a resistance."

Schutz is so committed to the program that he allows females to bite his arm once a week for the protein-rich blood they need to reproduce. Only females bite, and not out of hunger. Mosquitoes subsist on plant juices.

It's because some species can transfer diseases -- West Nile virus and canine heartworm among them -- that officials carefully identify each by its appearance and characteristics. Some bite at night, some at day; some travel great distances, some hover near home. Even their breeding water preferences differ.

The district's abatement program emphasizes educating residents -- "Clogged rain gutters can produce thousands of mosquitoes," Bass said -- and natural controls, primarily mosquitofish. The guppy-size fish happily dine on mosquito larvae -- as many as 500 a day. The district raises about 1 million of the fish per year, and offers them free to residents for use in mosquito breeding pools on their property.

The district, which is funded through property taxes and benefit assessments, also makes thousands of free service calls for residents wondering how to rid themselves of rats, mice, skunks or ground-nesting yellow jackets.

The newest concern is the discovery in Southern California of mosquitoes capable of spreading yellow fever and dengue fever. Schutz theorizes they hitchhiked their way to California in airplane cargo holds. They possess the ability to transfer the diseases from human to human after biting anyone who is infected. General Manager Craig Downs, with the district since 1981, said that's the biggest change in the agency's evolution -- its emphasis on disease control.

"When the West Nile virus arrived, it was the new invasive thing," he said. "Now we're seeing it again. Instead of (mosquito control) just being a standard-of-living thing, it's a true public health need."

Rest assured that the district is on the case, even if no one notices.

For more information, visit contracostamosquito.com. Contact Tom Barnidge at tbarnidge@bayareanewsgroup.com.