When someone coughs in Barry Prystowsky's office, you might say it is a hack heard 'round the world.

The New Jersey pediatrician is a "sentinel physician," part of a local-to-global flu-watching network that is gearing up to monitor the familiar viral infection this winter.

Besides about 2,000 volunteer sentinel doctors nationwide, the many-tiered network includes hospital emergency rooms, health centers, labs that test for influenza, schools and nursing homes. Over the next few months, they will all be gathering information about patients with flu-like symptoms.

This is no academic exercise. By examining and tracking the virus, global health officials decide what next year's vaccine should be. Resources can be redirected to patients -- and communities -- most in need.

The flu kills an estimated 500,000 people worldwide every year, including 36,000 on average in the United States.

Prystowsky, who has been in practice for 20 years, immediately thought of the flu when he examined a 14-year-old boy in his office last week. He noted the teenager's cough, runny nose, fever, body aches and diarrhea, took throat and nose cultures, and, as instructed by the Centers for Disease Control and Prevention in Atlanta, sent the specimens to a lab for confirmation.

If it is flu, that information will make its way to the New Jersey Health Department, the CDC and eventually the World Health Organization in Geneva, where it would become part of the global influenza picture.

"I think it's important for the state to have accurate numbers," said Prystowsky, one of several dozen sentinel doctors in New Jersey along with 71 in Pennsylvania. "They really have no way of judging what I'm seeing in the office if I don't tell them."

Flu information is compiled weekly throughout the flu season, which typically occurs between October and May, peaking in January and February. But the flu virus continually changes, so even with good numbers, it is impossible to anticipate where outbreaks will pop up or to create one foolproof vaccine.

Why bother counting cases?

At the highest levels, the data help determine whether this year's vaccine matches the current flu strain and inform predictions of what next year's strains are likely to be.

At the ground level, the numbers show how fast the flu is moving and in what direction, allowing more vaccine to be targeted to those areas.

"In a good year, when there's lots of flu vaccine," said Caroline C. Johnson, Philadelphia's director of disease control, the data "reinforce the need to get people immunized -- all the stragglers who didn't come in, all the doctors who were dragging their feet."

But this may not be a good year, as Johnson defines it. The U.S. vaccine allocation was cut in half because of problems at a supplier's plant in Liverpool, England. As the federal government scrambles to find more, public health officials hope for a mild season.

Once the flu virus is confirmed, each state informs the CDC, which is also part of the WHO disease surveillance network. Activated in 1952, it comprises 122 institutions in 83 countries.

Lab confirmation can take five days or longer, which is not much help to flu sufferers. So, in the short run, some doctors and others rely on "rapid tests" administered in the office with a throat swab. The test generally diagnoses the flu -- not the type or strain -- the same day, allowing doctors to prescribe antivirals immediately.

The flu is not the only thing being monitored. Each state determines which illnesses or conditions to watch. Many are standard from state to state, but there are variations.

"We want to know about every single case of measles, for example, because measles essentially has been eliminated from the United States and much of the Western Hemisphere," said William Schaffner, a government vaccine adviser and epidemiologist at Vanderbilt University in Tennessee and a former CDC investigator.

However, Lyme disease is a bigger problem in the northeastern part of the country and would be watched more closely here.

This year, given the vaccine situation, all eyes will be on influenza-like illness, the constellation of sniffling, fever and cough known as ILI.

Not all ILI is flu, which is more debilitating and can cause complications such as pneumonia as well as death, but the incidence of ILI is a pretty good barometer for how much actual flu may be in the region and the nation, said Eddy A. Bresnitz, New Jersey's state epidemiologist.

ILI surveillance is an early variation of what is known as "syndromic surveillance," real-time sorting of information from hospital emergency rooms to ferret out the early stages of an outbreak. Philadelphia's Health Department used this new alert system four years ago to detect possible anthrax or smallpox cases when the Republican National Convention was in town.

This year, it will be used to find the flu.

The city will collect emergency-room information every day from six hospitals, strip names from it and, using a CDC computer program, sort patient complaints by category to see who might have the flu.

The data are crude, and a faulty key stroke can cause false spikes or transmission errors. But, Johnson said, the public will benefit when the city is able to "get this information on a daily basis and call the hospital and say, 'Gee, we saw 10 people in your emergency room that sounded like flu yesterday. What do you think?' "

Ed Jasper, who has worked in Jefferson University Hospital's emergency room in Philadelphia for 20 years, said he loves the idea because it will help doctors catch trends early. "It's a great tool," he said.