Once feared as a coming plague, West Nile virus is proving to be a quirky cyclical infection that at present is almost absent from Massachusetts.

Only two birds have been found to be infected with the virus this seaason in the state, compared with 197 reported by the same date last year. No human cases of the virus have been diagnosed in 2004 in the commonwealth.

When it first appeared in the United States in New York City in 1999, the virus, still something of a mystery, aroused widespread paranoia. Some people hesitated going outside in affected areas and any dead animals might be reported to health officials as suspicious.

But five years later, West Nile virus has established a track record, and the mysteries about it are dissappearing.

The initial public reaction to the arrival of the virus "was certainly unrealistic, but not at all surprising. We treat most unusual events in our lives in that way," said Dr. Robert P. Hoffman, an infectious disease specialist at Mercy Medical Center in Springfield.

"In truth, the arrival of West Nile confirmed what most epidemiologists were predicting. West Nile virus is common in the Middle East. And because travel today is so much easier and more extensive than ever, we expected to see diseases traveling here from other parts of the world. It was inevitable."

Last year in the United States, West Nile virus killed 264 people and sickened nearly 10,000 others. This year, there have been 922 cases to date and 22 deaths. In Massachusetts since 1999, there have been 47 human cases and five deaths.

The peak season for the virus is from late August through September. Most people who have become infected have no symptoms. However, one in 26 people diagnosed with the infection in the United States to date have died from the illness.

At the heart of the illness's behavior is the interrelationships of the virus, humans, mosquitoes and birds, said Ralph J. Timperi, assistant commissioner of the state Department of Public Health.

The virus is passed to humans through the bite of infected mosquitoes, but since mosquitoes die in fall with the first frost, the virus would have no way to survive the winter if it were not for birds. Birds bitten by infected mosquitoes in the fall survive the winter and pass the infection back to mosquitoes hatched in the spring who bite them.

"When it's first introduced, none of the birds have ever been exposed so it spreads rapidly through the entire bird population. But in most birds, the infection doesn't cause mortality. And if they survive, they have an immunity that probably lasts for the entire life of the bird," Timperi said.

In time, there are few vulnerable birds left to be winter reservoirs for the virus, which creates a decline in infected mosquitoes and human cases the following season.

"But over time, the immune birds die out and the susceptible portion of the bird population increases. Then you get to a point where you have many susceptible birds and there is an opportunity for the virus to spread again, and that's when you get the highest number of human cases," Timperi said.

As evidence of this up and down cycle, by this date in 2003, Arizona had no human cases. This year, ther have been 307 in the state, the most in the nation. By contrast, South Dakota, which had reported 250 human cases by this date last year, has reported only 20 this year.

For the latest counts of West Nile cases click the link below to visit the Centers for Disease Control website where you will find the most up-to-date information.

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