FRIDAY, Feb. 10 (HealthDay News) -- It was just about three years ago that a strange new strain of flu first appeared in Mexico, then spread across the border to the United States and eventually much of the globe.

The H1N1 "swine" flu strain didn't behave like a "normal" flu, because it proved particularly dangerous to children and younger adults -- the very groups of people who usually have the strongest defenses against seasonal flu.

After a quiet couple of years, more cases of the pandemic H1N1 flu are circulating again where it all began -- in Mexico.

But infectious disease experts says Americans shouldn't be overly concerned.

In January, there were 1,623 cases of flu reported in Mexico, and 90 percent of those cases were H1N1 flu. There were also 32 flu-related deaths, all but three caused by the H1N1 strain, the Associated Press reported.

"It appears that H1N1 in Mexico is circulating at a higher level than in the United States," said Tom Skinner, a spokesman for the U.S. Centers for Disease Control and Prevention. "We have seen some H1N1 here in the U.S.," he added, but the more familiar H3N2 strain is predominating here this winter.

The flu season in the United States has also gotten off to a slow start, Skinner said, but it's expected to pick up in the coming weeks. "Our season usually peaks sometime in February and it's not too late to get a flu shot," he said.

The flu shot for this year -- and last year -- includes protection against the H1N1 strain, Skinner noted.

Infectious disease expert Dr. Marc Siegel, an associate professor of medicine at New York University, doesn't think there's much for Americans to worry about, given the situation in Mexico.

"First of all it [the H1N1 virus] turned out to be a very mild virus," he said.

"One of the reasons Mexico saw a severe outbreak compared with us in 2009 is that initially you see more lethality and more morbidity. Then, as the virus spreads, it usually becomes less severe -- that's traditional," Siegel added.

Also, flu pandemics tend to follow a pattern like the one taking place now. They come in "waves" and there are always additional "waves" in the second and third year after flu strain's initial appearance, he said.

"It just becomes one of the circulating viruses," Siegel said.

It's unlikely that another severe outbreak of H1N1 would occur in the United States, he said.

"We have developed a 'herd immunity' through previous exposure to the virus and vaccination, so it slows the spread," he said.

According to the CDC, one reason that children and young adults were more vulnerable to the H1N1 strain back in 2009 is that this strain hadn't circulated widely since the early half of the 20th century. As a result, CDC studies found that no children and very few adults younger than 60 had existing antibodies to the 2009 H1N1 flu virus. Curiously, about one-third of adults 60 and older are thought may have antibodies that may help protect against the virus.

More information

To learn more about the H1N1 flu, visit the U.S. Centers for Disease Control and Prevention.