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A waiter stands at an empty restaurant in Cancun, Mexico, on Friday. The swine flu epidemic has badly hurt Mexican tourism. Students visiting Mexico on spring break brought the virus to the United States, where they then infected schoolmates and friends, experts say.

WASHINGTON — The swine-origin H1N1 flu virus that burst into public consciousness a month ago is starting to behave like a mixture of its infamous, pandemic-causing predecessors.

It seems to have a predilection for young adults as did its notorious ancestor, the 1918 "Spanish" influenza. Many of the young victims who have become deathly ill turned out to have other medical problems – a phenomenon first clearly seen with the 1957 "Asian" flu. H1N1 is spreading easily in North America but sputtering in Europe, just as "Hong Kong" flu did in 1968. And like the mini-pandemic of "Russian" flu in 1977, some people appear to have a degree of immunity.

Exactly how swine flu fits into the pantheon of pandemics will not be known for a while. It will take months – and many more victims – for its full personality and behavior to emerge. But one thing is clear: This is a lot more than just seasonal flu out of season.

After a brief moment when news of the outbreak in Mexico made swine flu look like a horseman of the apocalypse, public health officials have spent much effort reassuring people that most of the time, this virus causes a mild illness that can be ridden out at home.

Yet, officials at the World Health Organization, the Centers for Disease Control and Prevention and elsewhere do not want the public to get blase.

Pandemic flu strains – and this new H1N1 strain is all but certain to cause the 21st century's first pandemic – are unpredictable.

"Our message to everybody is, of course, do not over-worry about these things, (but) it is important to know it is serious," the WHO's Keiji Fukuda said last week.

Perhaps the most worrisome features so far are the number and severity of cases in teenagers and young adults. The average age of the confirmed and probable cases is 15 years. Two-thirds are younger than 18. There are two theories for what is happening.

One is that students visiting Mexico on spring break were the chief "vectors" bringing the virus to the United States, where they then infected schoolmates and friends. The other is that young people are especially vulnerable for some reason.

"As we get farther and farther in, are we going to be able to choose between these two hypotheses? Sure we are," Joseph Bresee, the CDC's chief flu epidemiologist, said late last week. But it may take two or three months.

The pattern has not changed much now that there are 5,000 laboratory-confirmed infections and probably more than 100,000 overall.

Determining the true age distribution is crucial, as it will help set the policy for who should be first in line for vaccines and how to ration antiviral drugs if they are in short supply.

A closely related question is whether the illness tends to be more serious in younger age groups.

In the U.S., the familiar seasonal flu causes about 8,100 deaths a year directly and contributes to about 36,000 more in people with lung or heart problems. Ninety percent of those deaths occur in people 65 and older.

Compared to seasonal outbreaks, all flu pandemics cause a higher percentage of severe cases and deaths in younger groups. Although the overall mortality rate from the current swine flu is low, this trend is already apparent.

Last Thursday, when Fukuda announced that the global death total was 65, he noted that "half of them are healthy people who have no predisposing conditions. This is a pattern different from what we see with normal influenza."

There have been too few deaths in the United States to draw any conclusions. But of the 173 people who have been sick enough to be hospitalized, more than half are in the 5-to-24 age group.

In the 1918 pandemic, which killed at least 50 million people, nearly half the deaths were in people 20 to 40 years old. In comparison, 36 percent of deaths in the 1957 pandemic were of people younger than 65, and 48 percent...