Though late fall is approaching, mosquitoes stay active until the first hard frost--which might be later than usual this year, thanks to the unseasonably warm weather in many parts of the country. And mosquitoes not only make you itch but can carry West Nile virus and several other viruses that can cause, among other things, encephalitis, a potentially deadly brain inflammation.

West Nile virus can cause fever, meningitis, and even death. Indeed, 49 deaths from the virus were reported in the U.S. through mid-September 2007, according to the federal Centers for Disease Control and Prevention. The most cases have been reported on the West Coast and in the Great Plains and Rocky Mountains, though cases have been reported in 39 states so far this year.

Less-well-known mosquito-borne viruses include La Crosse encephalitis, St. Louis encephalitis, western equine encephalitis, and eastern equine encephalitis. All can cause mild flu-like symptoms. And eastern equine encephalitis can be deadly. Though there have been only about 220 confirmed cases of that disease from 1964 to 2004, nearly a third of those infected die, and half of those who survive suffer permanent neurologic damage.


PROTECTING HOUSE AND YARD

The key to keeping mosquitoes out of your house and yard is denying the females comfortable places to lay their eggs. It takes energy for mosquitoes to fly. If you're breeding them in your yard, you're the first one they're going to bite, say mosquito-control experts.

House mosquitoes are drawn to muddy, murky bodies of water. Keep your yard and house free of containers filled with standing water and you've won half the battle.

Keep your rain gutters scrupulously clean; Culex mosquitoes, which can carry the West Nile virus, are partial to waterlogged, decaying leaves.

Don't allow water to accumulate for more than a few days in any container. Mosquitoes can multiply in untended dog bowls, birdbaths, birdfeeders that collect water, saucers under flowerpots, wheelbarrows, children's wading pools, and the water that collects in swimming-pool covers. But, mosquitoes will not breed in above-ground or in-ground swimming pools that are kept clean and properly chlorinated.

If you see a mosquito indoors, kill it immediately. You can swat it with your hand without fear because it takes a bite to transmit West Nile virus.

Research shows that dogs don't get sick when infected with West Nile virus, and cats get a brief, mild illness at worst. Neither is likely to transmit the disease.

Electric mosquito traps catch and kill some mosquitoes outdoors but they're costly and are not safe indoors.


PREVENTING BITES

The most effective way to avoid bites is to stay inside when mosquitoes are active. For Culex and most mosquitoes, that means at dusk and throughout the evening.

When you do go out at those times, wear light-colored, loose-fitting clothing that covers as much of your skin as comfort permits. Mosquitoes will bite right through tight-fitting fabrics. Avoid perfumes and aftershaves, which attract some mosquitoes.

Wear a repellent on exposed skin--ideally, one with protection that lasts no longer than the time you're going to be out. Products containing at least 30 percent deet work best, protecting for an average of at least five hours, according to our recent tests. The U.S. Environmental Protection Agency has found deet to be safe when used according to directions (not under clothing, on an open cut, or with sunscreen). We recommend using the lowest concentration that works, especially on kids, and never using deet on infants.

If you don't like deet's scent or feel, you could try Repel Plant Based Lemon Eucalyptus, the highest-scoring botanical product in our tests, or Cutter Advanced with 7 percent picaridin, though our most recent test of that product found it provided only limited protection.