Millennials, beware.

The H1N1 strain of flu, commonly known as the “swine flu,” has made a return in the 2013 to 2014 flu season, and has proven more lethal to the young than the usual flu virus.

“This is the first season that the virus has circulated at such high levels since the (2009) pandemic,” the Center for Disease Control (CDC) reported.

60 percent of people hospitalized because of the flu so far are between the ages of 18 to 64 years old.

According to Forbes, “This is a big change from past flu seasons, in which adults under 65 have made up just 35-45 percent of those seriously ill enough to be hospitalized.”

That the flu has been less choosy this year – hitting not just the elderly and infirm – makes sense when remembering the characteristics of the 2009 flu season.

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Symptoms of the swine flu. Photo courtesy http://en.wikipedia.org/

In 2009, when the initial H1N1, swine flu epidemic broke out, CDC found that the most cases of H1N1 influenza occurred among those between five and 24 years of age, with 41 percent of hospitalizations occurring among older children and young adults.

The anomaly of 2009 was that the young were disproportionately struck by the flu.

The CDC reported, “only 13% of hospitalizations had occurred in people 50 years and older, and there were few cases and no deaths in people older than 65 years, which was unusual when compared with seasonal flu.”

Despite the increase in swine flu cases in 2013 to 2014, this flu season has not come close to reaching the level of 2009, which induced the World Health Organization to declare a global pandemic of H1N1.

This is due in part to the development of a vaccine to fight this particular strain of flu virus. At its appearance in 2009, swine flu was a new phenomenon, a type of influenza not yet encountered among humans. Subsequent research produced vaccines and increased knowledge of the virus’s spread.

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Ouch. Photo courtesy http://i.telegraph.co.uk/

Even with a vaccine, millennials remain at high risk for acquiring swine flu in 2014; Young adults are least likely to acquire a flu vaccine, but most at risk to catch H1N1.

The CDC found that the young may have lower resistance to H1N1 because they lack a “cross-reactive antibody,” which one-third of adults over 60 had in 2009.

Because young adults frequently are not vaccinated against the flu, they still lack the ability to naturally fight the swine flu virus.

For the week of February 9-15, the CDC reports that cases of the 2009 H1N1 virus continue to dominate the 2014 flu season.

The CDC also cautions that “Flu activity is still elevated and likely to continue for weeks in the United States.”

The Washington Post reported the number of deaths from the flu so far this season on February 19.

“The outbreak has been especially severe in California. There have been 243 deaths of residents younger than 65 so far this year. An additional 41 cases were reported but have not been confirmed,” the Post reported. “In the 2012-13 season, there were 26 deaths by this time, and in the 2011-12 season there were nine deaths. In the 2009-10 season, there were 527 deaths.”

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This is what the swine flu virus looks like under the microscope. Photo courtesy cdc.gov

Outside the United States, recent concerns have been raised about a strain of avian flu found in an elderly woman. This strain of “bird flu” has not been seen before this month.

There is ongoing global concern about H7N9, a form of influenza virus that has killed around a quarter of those infected, according to BBC News.

These deadly strains have less immediacy to the US, but are still of concern to the global community as a whole.

For a regional map representing “Flu-like illness activity level” for the week ending Feb. 8, view this CNN article.

More info on what’s in the 2014 flu vaccine and its efficacy here.

Writer’s note: I am a millennial, and I have not gotten a flu shot in years.

Feature photo screen shot capture: http://thumbs.dreamstime.com/z/swine-flu-11079273.jpg