(The New York Times News Service) -- As isolated cases of swine flu emerged in spots as far-flung as Israel and New Zealand, disease trackers scurried Tuesday to understand the true threat posed by the novel virus, which has now made at least 64 Americans sick and placed health authorities around the world on high alert.

The germ has stoked widespread fears because humans have no immune protection against it, and in Mexico, where it appears to have originated, even previously healthy young adults have died. And with the health crisis emptying the streets of Mexico City, closing a school in New York, and prompting President Obama to ask Congress for $1.5 billion in emergency aid, federal health authorities were unvarnished in their prediction about what future days and weeks portend.

"As this moves forward, I fully expect that we will see deaths from this infection" in the United States, said Dr. Richard Besser, acting director of the US Centers for Disease Control and Prevention.

Already, in New York, the roster of prep school students with flu-like symptoms grew by hundreds of names Tuesday, and the city's health commissioner said he believed they were probably stricken with swine flu.

Not far away, at a public school, more than 80 special education students called in sick, resulting in the decision to shut and disinfect classrooms.

Swine flu, said New York health commissioner Thomas Frieden "is here and it is spreading."

Still, much about swine flu remains cloaked in mystery. Among the most pressing questions is why the virus caused hundreds of people to fall gravely ill in Mexico while the infected in the United States experience far milder symptoms. And, too, disease specialists want to know how easily the germs travel from one person to another.

The answers to those questions could profoundly influence decisions being made in coming weeks by health authorities in the United States and globally, who have abandoned any hope of containing the virus within geographic boundaries.

Disease detectives are scouring Mexico City and the five US states with confirmed cases, and their findings will help determine, for example, whether a vaccine against swine flu should be concocted and whether closing schools when cases are detected makes sense medically and economically.

"Clearly, the horse is out of the barn right now," said Dr.

Wayne A.

Marasco, a flu researcher at Dana-Farber Cancer Institute. "How big is the problem? Is it going to be a persistent infection or burn out?

These are the questions that healthcare officials are trying to address right now."

In Massachusetts, the more urgent question Tuesday was whether four elementary school children with flu-like symptoms actually carry the virus that causes swine flu. Samples from the children - two each in Lowell and Spencer - have been sent to government labs for genetic fingerprinting, local health authorities said.

In Spencer, siblings in the second and third grade at the Lake Street School had just returned from a family vacation in Mexico and fell ill Tuesday, said Ralph Hicks, superintendent of the Spencer-East Brookfield Regional School District. "I had the principal take the rest of the children out of the rooms, the custodian disinfected surfaces - things kids would touch - desk tops, doorknobs, and this evening the entire building is being disinfected," Hicks said Tuesday night.

A spokeswoman for the Massachusetts Department of Public Health said Tuesday that no cases of swine flu have been confirmed in the state, nor has any other New England state reported positive results. Federal authorities said all 64 patients with confirmed cases of the disease reside in five states: California, Kansas, New York, Ohio, and Texas, with three people requiring hospitalization in California and two in Texas.

They range in age from 7 to 54 years old, with an average age of 16 - evidence reflecting that the majority of confirmed cases are from the New York preparatory school.

Reflecting the fluid nature of the threat, wire services Tuesday afternoon reported that the Los Angeles County coroner was investigating whether the germ was implicated in the deaths of two men, one who was 33, the other 45.

By early evening, swine flu had been ruled out as the cause of one death and discounted as the possible source of the other.

Federal medical detectives have fanned out across North America, hoping to better understand the virus, its victims, and the environments where both dwell.

One of the first, and most basic, questions: Just how many people in Mexico are infected?

"The reports from Mexico indicate there may be as many as 150 fatalities because of this disease," said Dr. Jeffrey Kahn, an infectious disease specialist at the Yale School of Medicine. "The question is, is that the tip of the iceberg or is that the whole iceberg?"

Researchers are asking whether the relative mildness of cases in the United States means that the genetic machinery of the virus somehow changed as it leaped across borders, making it less virulent. Or, instead, do the deaths in Mexico represent only a small fraction of total cases, suggesting that the virus may be less fearsome than some believed but capable of making lots of people mildly ill?

"Suppose that the virus is severe in a low proportion of people," said Marc Lipsitch, an epidemiologist at the Harvard School of Public Health.

"Then if you look for only severe cases and there are a lot of them, you'll notice them and miss the thousands or tens of thousands of other, milder cases.

"My best guess is that we will start to see severe cases in the United States and that once they are able to look in a systematic way, they will probably be able to see there are a lot of mild cases in Mexico."

When disease clusters are identified, such as the teenagers stricken at the New York prep school, investigators can use mathematical models to determine what percentage of people who had contact with a virus carrier developed symptoms.

"That attack rate is a measure of how effective the virus is in moving from person to person," said Dr. Alfred DeMaria, the state epidemiologist in Massachusetts.

Disease specialists will also examine whether there were differences in the treatment provided to patients - and when - and how those patients ultimately fared.

With those findings, top federal health authorities will determine their next steps, with an eye to whether the time of swine flu's arrival, as the weather turns warm and inhospitable to the flu virus, will buy the nation time.

"There are many factors conspiring in our favor," said Dr. Ian Lipkin, an epidemiologist at the Columbia University Mailman School of Public Health.

"It doesn't mean we can be complacent, but this is a bullet we may have dodged in the Northern Hemisphere."

Copyright 2009 The New York Times News Service. All rights reserved.