California and parts of the Bay Area are expecting the current West Nile virus season to be the worst in at least five years, with almost twice as many cases of the viral infection in humans so far compared with last year.

Contra Costa County officials announced the first Bay Area case on Wednesday, in a woman who was infected in mid-August and is recovering now. With the peak reporting weeks still ahead, five people in the state have died from the virus, including a 74-year-old Placer County man whose death was reported Thursday.

Beyond California, the United States is in the grip of what may turn out to be the worst outbreak of West Nile since the virus arrived in the country 13 years ago. Although the national outbreak may have reached its peak, public health authorities say they won't have full reports on the extent of the epidemic for another few weeks.

More than 2,600 cases of West Nile virus infection have been reported nationwide this year - 40 percent of them in Texas, according to the U.S. Centers for Disease Control and Prevention. In California, there have been more than 90 cases so far.

"We're still getting a lot of cases reported to us - 17 cases just this past week," said Vicki Kramer, chief of the vector-borne disease section of the California Department of Health Services. "I think we can expect to see more cases before the season is over."

West Nile virus was first identified in Uganda in the 1930s, but it didn't arrive in the United States until 1999. Over the next several years, it spread across the United States, and the first human case was reported in California in 2003. The Bay Area West Nile season typically spans from June to late September, but this year the virus was first detected in April, and the season can extend to the end of October some years.

The virus is carried in birds, which are bitten by mosquitoes that then bite humans. After a human has been bitten by a mosquito, it can take two to 12 days for symptoms to show up. The virus is not passed from human to human.

From 75 to 80 percent of people who are exposed to the virus never have symptoms. Most of the remaining will have flu-like symptoms - such as fever, headache, body aches and swollen lymph glands - that last for up to a week.

Dangerous conditions

But in roughly 1 percent of cases, the virus will invade patients' brain or central nervous system and cause meningitis or encephalitis, dangerous conditions that can cause muscle weakness, permanent paralysis and death.

There is no treatment for West Nile virus, and patients usually recover on their own. Individuals who develop symptoms of neurological infection like confusion or muscle weakness should seek medical attention immediately.

"There's not much we can do but let the disease run its course," said Dr. Charles Chiu, an infectious disease expert and head of the viral diagnostics laboratory at UCSF, who treated the first case of West Nile virus in California in 2003.

This year's national outbreak is especially alarming because there have been 1,400 cases of the neuro-invasive disease reported so far - the largest number since the virus was first found in the United States, according to a CDC report released this week.

Based on that number, CDC authorities believe that roughly 200,000 people in the United States have been infected with West Nile - far more than the reported cases - and as many as 50,000 people have been sickened by it.

"It seems like all of the conditions were just right for an outbreak this year," Chiu said. "There were some large outbreaks in the U.S. in 2002 and 2003, but this year is already on pace to outstrip even those outbreaks. The good news is it does appear that we're near the tail end of this."

Mosquito-control experts say the cause of this year's unusually large outbreak - both nationally and locally - isn't entirely clear, but is likely related to abnormal weather conditions last winter and this summer.

California's winter

A warm, dry winter may have caused mosquitoes to come out of hibernation earlier than usual, which would explain, in California, why the first evidence of West Nile in birds and mosquitoes was found almost three months ahead of normal.

In the South, a hot summer may be to blame for rapid reproduction of both the insects and the virus they carry. Mosquitoes can go through an entire life cycle in just five days in hot weather.

Ironically, vector-control experts said, a dry year can actually cause an increase in mosquito populations. Mosquitoes often lay eggs and develop into adults in shallow, stagnant water, and rain storms, especially in spring and early summer, are key to washing away some of those pools. Plus, with fewer standing pools of water to draw birds and mosquitoes, both creatures tend to congregate in heavily trafficked areas - leading to easier spread of the virus.

The West Nile season may not be over for another six weeks, so Bay Area residents need to remain vigilant against spread of the virus, said Deborah Bass, a spokeswoman for the Contra Costa Mosquito and Vector Control District. She said the district is still getting near-daily reports of dead birds that test positive for the virus, and is spraying mosquito pesticides to kill off disease-carrying populations.

"Oh look, as we speak, another bird just came up on my screen," Bass said during a phone interview on Wednesday. "We don't know how many people are going to be diagnosed with the disease this year."