"We've had many more [cases] in years past," said Nassau's commissioner of health services, James L. Tomarken.
Last year, Suffolk reported a total of four confirmed cases of the virus and no deaths, and Nassau reported 16 confirmed cases and one death. But two years ago, Suffolk had 25 cases and three deaths, and Nassau had 34 cases and two deaths.
"It's difficult to predict year to year where it will be heavy because a lot of factors change," said John Emery, a state Health Department spokesman. Health officials have not been able to completely explain why the Island has fared so well this summer.
Nationally, West Nile virus cases are up 40 percent just since last week, and may rival the record years of 2002 and 2003, federal health officials told The Associated Press.
So far this year, 1,590 cases and 66 deaths nationally were reported to the Centers for Disease Control and Prevention, the AP said.
States hit hard by drought and scorching temperatures have also been besieged by mosquitoes, carriers of West Nile.
Tomarken said the drought could be "a major factor" behind the trend: "When it's hot and dry, that's also a time for mosquitoes."
Less rain means more stagnant water, mosquitoes' ideal breeding grounds, said Emery, who cautioned that there are likely other reasons for the rise of West Nile in the South.
The factors behind the numbers are unclear for two major reasons -- the variety of factors affecting mosquitoes and the virus' spread and the lack of extensive studies, said Marm Kilpatrick, a disease ecologist and West Nile virus expert at the University of California.
"If you hired me to do a scientific study . . . I'd try to find out what happened to the mosquitoes, what happened with the birds and what's happening to the people," Kilpatrick said. "What's their kind of previous immunity and what's their differential behavior in terms of protective behavior, bug spray and going outside at dusk?
"Drought followed by a wet spring and warm summer and that gives you this amazing West Nile epidemic -- it's a great story, right? The challenge is that people want to point to that when it lines up, but then when it doesn't line up, people kind of ignore that fact," he said.
He said one theory is more Long Islanders have developed some immunity to West Nile virus. Also, Suffolk and Nassau counties have sprayed against West Nile-carrying mosquitoes for years, the scientist said.
Budget concerns and residents' health concerns have prevented Texas, for example, from spraying until this year.
Kilpatrick said he and other researchers are working with the CDC to analyze 10 years of West Nile-related data on mosquitoes, birds and people to see what drives up infections.
One question is when warmer temperatures stop being good for mosquitoes, said Kilpatrick, who years ago used data from many counties, including Suffolk, to identify the mosquito species responsible for most West Nile cases in humans. That was Culex pipiens, a hot weather species that breeds in stagnant water, comes out at dusk and is more effective at passing the virus to humans than Culex restuans, a cooler weather mosquito that bites mammals but is less effective at infecting people, Kilpatrick said.
Thirteen people have fallen sick due to the virus in New York State this year, health officials said. Last year, there were 44, including two deaths, one in Nassau.
New York City was where the virus was first detected in this country in 1999, and shortly thereafter it was found on the Island.
The first Texas case cropped up in 2002, Kilpatrick said.