West Nile may be down, but is it out?

West Nile virus, which made its US debut nine years ago in New York and proceeded to spread coast to coast, appears to be in retreat this year, resulting in substantially fewer cases of human illness.

A Globe analysis of federal figures shows that compared with the same period in 2007, half as many people this year have developed the most serious neurological complications of the mosquito-borne malady. Through Tuesday, there had been 11 deaths nationally - less than one-tenth of the tally for all of last year.

The only New England state reporting cases is Connecticut, where five people have fallen ill. And in the regions that have disproportionately borne the burden of West Nile in recent years, especially the Plains states, the decline in human cases has been particularly precipitous: By mid-September last year, Colorado had reported a total of 329 cases, including those with mild and more severe symptoms. This year, thus far: 64, according to a state epidemiologist.

And in another telling sign, the number of West Nile infections detected in blood donations has plummeted by about half this year.

While disease specialists are loath to declare the threat over, they do hint at the possibility that West Nile, like other viruses spread by mosquitoes, may settle into a cycle. That could mean occasional peaks in human infections followed by quiet years. "I don't think anybody knows whether we're going to stay at these sustained lower levels," said Dr. Marc Fischer, a top West Nile scientist with the US Centers for Disease Control and Prevention in Fort Collins, Colo.

Since arriving in the United States, West Nile has left thousands of people ill, hundreds dead, and served as a reminder of the threat presented by newly emerging infectious diseases.

Health authorities in Massachusetts and other states still issue regular warnings about the dangers posed by the viral infection, and in the town of Halifax this summer, authorities made it illegal to let water stand for too long, lest it become prime breeding terrain for mosquitoes. "It's far too soon to say that we're out of the woods yet," said Dr. Bela Matyas, an infectious disease investigator at the Massachusetts Department of Public Health.

West Nile has proved to be anything but predictable, emerging with fury one year in one region and subsiding the next. Disease trackers remain befuddled by its trajectory, uncertain where the virus will exact the most damage.

"The one thing we've learned about West Nile is that it's so surprising," said Lon Kightlinger, state epidemiologist in the South Dakota Department of Health. "It's not like influenza where we just cough on each other, and it gets spread that way."

Instead, West Nile is a reflection of the intricate dance among human, mosquito, and bird, not to mention the vagaries of rain and heat. There has to be just the right balance of forces for a cascade of infections to start.

Bugs get all the blame for spreading the disease, but birds are the primary reservoir for the virus, either becoming infected by mosquitoes or infecting the insects when they hunt for blood.

Scientists speculate that because so many birds have been exposed to the virus, they have either developed mass immunity - or been killed. It's a lot like forest fires: If enough trees burn in one season, then a subsequent fire can't explode into a massive blaze because not enough kindling remains.

"Usually, when a disease like West Nile is introduced, the number of cases is higher because there's a much more susceptible population, especially the bird population," Matyas said. "Now that every part of the country has had the disease for several years, we're beyond the point of the initial impact."

Consider the case of Colorado this year. Cool weather settled into the state at the start of mosquito season, which meant the breeding season got off to a slow start. Later, there was the heat mosquitoes favor, said John Pape, epidemiologist with the Colorado Department of Public Health and Environment. But to breed, the insects still need water. "And there was just no water," Pape said.

Another possible factor in this year's West Nile decline: People are actually heeding public-health messages to slather on bug repellent and avoid being outdoors during peak hours for mosquito activity. "We would love to think that mosquito control efforts and personal protection efforts are working," the CDC's Fischer said, "but I think that's the least likely factor playing into it."

Stephen Smith can be reached at stsmith@globe.com.