WASHINGTON – Pull out the bug spray: West Nile virus is beginning its summertime assault.

Infected mosquitoes are known to be buzzing in 16 states so far, and five people nationwide are already battling the illness’s most severe form. Scientists fear the rubble-strewn Gulf Coast in particular is ripe for a bad outbreak.

How bad this year will be depends on the weather. Anywhere that’s especially hot and dry should watch out.

It also depends on birds – robins and house sparrows, to be exact. Forget the dying crows that became notorious in West Nile’s early days. How mosquitoes feed on these smaller backyard birds seems more important in determining how much virus circulates in communities – especially in July and August, the disease’s worst months.

Since it emerged in New York City in 1999, West Nile virus has spread from coast to coast infecting a surprising 1.2 million to 1.3 million people, estimates Dr. Lyle Petersen of the Centers for Disease Control and Prevention, the country’s leading West Nile specialist.

Only Maine and Washington have diagnosed no ill people yet, despite finding the virus in mosquitoes and other animals in summers past.

The Maine Center for Disease Control and Prevention recently announced the opening of its West Nile hot line and is taking reports of dead birds. The number to call to report dead birds found in Maine is (888) 697-5846. MCDC is currently not testing any birds for the virus, but will begin that testing within the next few weeks. More information about West Nile in Maine, including safe handling of dead birds, is available online at www.mainepublichealth.gov.

About one in five people develops symptoms, and fewer get the life-threatening disease. Still, West Nile has killed almost 800 people in the U.S. in that brief period, and caused severe neurologic illness, meningitis or encephalitis, in more than 8,300. Others are left with polio-like paralysis.

Even the less severe West Nile fever is “really quite a horrible kind of illness,” says Petersen. He caught the disease himself in 2003 – spending a week in bed and a month afterward battling bone-deep fatigue – and he worries that people don’t take the threat seriously enough.

“I guarantee it’ll ruin your summer.”

Scientists can’t predict where the virus will strike each year, but recent research shows:

. West Nile virus grows faster and more plentifully inside a mosquito’s body when it’s really hot.

. Just about any mosquito can carry West Nile, but the biggest carrier, the Culex breed, thrives in small amounts of nutrient-rich water, like the muddy puddles left when ponds dry out in a hot, dry summer. Even drops left under dense leaves in irrigated fields or in the bottoms of flower pots can be enough.

. Hurricanes don’t spur West Nile. Instead, heavy rains temporarily flush out the tiny pools where mosquitoes have laid eggs. Yet the CDC fears that New Orleans and the Gulf Coast are ripe for a surge in West Nile this summer because of the rubble left by Hurricane Katrina last year, full of water-collecting crevices that make perfect mosquito breeding grounds.

. Birds are the incubators key to West Nile’s spread. Some, like crows, die quickly when bitten by an infected mosquito. Others, particularly robins and house sparrows, build up high levels of the virus without dying off, able to then infect more mosquitoes. When it’s hot and dry, those birds compete with mosquitoes for the same scarce water supplies – close contact that spurs more bug bites.

. And scientists who checked the stomach contents of live mosquitoes found that one big West Nile carrier tends to mostly bite robins until June, when those birds finish nesting and start migrating. Then, rather than filling the gap with just other birds, the bugs bite more people in late summer.

Already this year, doctors are reporting the neurologic form of West Nile in five people in California, Colorado, Mississippi and Texas. All donated blood is tested for West Nile, to prevent transfusions from symptom-free but infected people, and the CDC hopes those tests will also act as an early warning signal of impending outbreaks – in addition to mosquito testing that has uncovered infection in 16 states and counting.

Older people and organ transplant recipients are most at risk of life-threatening disease, while, mysteriously, the polio-like muscle weakness tends to strike people in their 30s, 40s and 50s.

But the sad truth is that West Nile can strike anyone, and there’s no vaccine yet. Hence the CDC wants people to use mosquito repellents as routinely as they do seat belts, so they’ll be covered wherever and whenever the virus pops up. The best advice:

. Spray on a good repellent whenever you go outdoors, either that old standby DEET or two recently proven alternatives, picaridin or oil of lemon eucalyptus.

. Make it harder for mosquitoes to breed in your neighborhood. Don’t let water collect in flower pots, buckets, old tires or wading pools. Clean out birdbaths weekly, and clean clogged gutters.

. Make sure window screens have no holes.

. And consult a doctor for such symptoms as high fever, severe headache, confusion or difficulty thinking, stiff neck, severe muscle weakness or tremors.