SAN FRANCISCO -- The next time you scratch an itchy red welt on your leg or back, public health officials say it may not be the fault of Northern California's abundant poison oak.

Infestations of bloodsucking bedbugs are on the rise throughout the San Francisco Bay Area, and the nocturnal pests are nesting in mattresses and cushions in hotels, homes, cars, buses, airplanes and movie theaters.

SLIDESHOW: Bed Bugs

Nearly 300 bedbug infestations were reported in San Francisco last year, more than double the number in 2004.

Exterminators say the majority of local infestations are in inexpensive motels, but they've performed pesticide sprays at luxury resorts and suburban mansions.

It costs as much as $5,000 to eradicate bugs from a large, single-family home.

"We never treated bedbugs until 2002. Now we have a dedicated bedbug crew working on this every day," said Luis Agurto, president of Pestec Inc. in San Francisco.

In August 2006, NBC11 reported a bed bug infestation at some dormitory rooms at San Jose State University. Some say foreign students may have unknowingly carried them from other countries where bed bugs are still a problem.

Some say foreign students may have unknowingly carried them from other countries where bed bugs are still a problem.

Also in August 2006, a couple from Chicago filed suit against a hotel near the San Francisco Airport after they were allegedly bitten during a stay last year, according to the Mercury News.

All 50 states are reporting increased outbreaks of bedbugs, rust-colored crawlers about the size of a lentil. Exterminators say they're tough to eradicate. Left alone, a few bedbugs can create a colony of thousands within weeks, and they can survive for months without any nourishment.

The wingless insects lay eggs in beds, clothing and even light fixtures and sockets. They hatch in about 10 days, then begin feeding on human and animal blood at night. They leave small, red bumps on victims' skin -- and sometimes they also leave small blood stains on sheets. They don't pass diseases to humans.

The San Francisco Chronicle contributed to this report.