Two years do not a trend make, but it does seem that with each passing summer, the number of human West Nile virus cases tends to decline. That said, there is no reason to relax. No one can predict reliably from year to year whether this, or any other mosquito-borne viral illness, might come back to infect humans, says Jim Miller, West Nile coordinator for New York City. "West Nile has been well documented since it was introduced here two years ago. It's a totally new virus in this part of the world, and I don't think we've reached a point yet where it's stabilized."

Many factors affect the spread of West Nile: which particular mosquito species are infected, weather conditions, rain--especially conditions that cause areas to flood and then dry up again--and how much pesticide is sprayed. This year, epidemiologists are on alert in all 48 contiguous states,...

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