'Once a virus such as West Nile reaches an area, it will stay forever in low levels. It may even die out for a period of time. Then, if the key factors all come together, particularly certain weather patterns, a major epidemic can occur.'

So far, more than 1,000 West Nile diagnoses have been reported, most of them within the past month. Illinois officials counted 56 new cases just last week, nearly all of them in the Chicago area. The state's total this year now stands at 105.

California has reported the most West Nile virus infections for the second year in a row. This year's tally is 521 so far.

'Definitely Here to Stay'

Although West Nile virus activity generally peaks in August and September, floodwaters and rain from Hurricane Katrina have heightened concern in affected states.

Last year, the US Centers for Disease Control and Prevention reported of 2,749 human cases of West Nile virus, with 900 of those taking the form of the serious "neuroinvasive disease." There were 88 deaths.

Though the incidence of West Nile virus has declined since 2003, regional outbreaks will occur on a regular basis, according to Jonathon Day, PhD, a medical entomologist with the University of Florida.

"West Nile Virus is definitely here to stay in the United States," Day says. “Once a virus such as West Nile reaches an area, it will stay forever in low levels. It may even die out for a period of time. Then, if the key factors all come together, particularly certain weather patterns, a major epidemic can occur.”

Other mosquito-borne diseases, such as Eastern equine encephalitis (EEE), are a threat public health. Although it is rare, EEE kills nearly half of those it infects. This year, a young girl and an elderly man died from EEE in Massachusetts. A 63-year old woman from the same area remains ill. Four cases of EEE have been diagnosed in New Hampshire.

DEET-Based Repellenets

Three steps experts frequently recommend for preventing mosquito bites are avoiding spots where the insects are plentiful; wearing protective clothing; and applying insect repellents, particularly those containing DEET.

"DEET has long been the gold standard for effectiveness against mosquitoes, ticks and many other insects, and DEET has been used by consumers with confidence for nearly 50 years," said Susan Little, executive director of the DEET Education Program, which is sponsored by companies that manufacture DEET and repellents.

DEET offers superior protection against insect bites. according to a New England Journal of Medicine article published in 2002. The study compared protection against mosquitoes provided by numerous repellents.

"Our study shows that only products containing DEET offer long-lasting protection after a single application," the authors state.

Based on that study and other research, the CDC and the American Academy of Pediatrics have recommended DEET-based repellents to protect against mosquitoes and ticks.

This year, CDC revised its recommendations and now tells consumers to use products registered with US Environmental Protection Agency, which reviews safety and effectiveness. Of the EPA-registered product types, CDC says "two have demonstrated a higher degree of efficacy in the peer-reviewed, scientific literature." Those are DEET and picaridin.

Read the Label

No repellent products in the world have been used more extensively than those with DEET as the active ingredient, according to Little.

"Read label instructions carefully and follow them," she said. "Apply repellents to exposed skin. To apply to your face or your child's face, put product on your hands and them hand-apply to the face. This helps keep the product out of the eyes and mouth."

Repellents containing up to 30 percent DEET can be used on children over two months of age, says the American Academy of Pediatrics. DEET should be used on clothing only if it is thin and tight-fitting enough for mosquitoes to bite through, Little advises. Some products can damage synthetic fibers but are fine to use on cotton and other natural fibers.

The most commonly reported problem with DEET-based products is stinging from repellent in the eyes, which is quickly remedied by flushing the eyes with water or saline solution, according to the product's precautionary statements on the label.