(NBC News/Maggie Fox) — People's birth years can affect their risk of catching certain strains of influenza — probably because their first case of flu somehow sets their immune system, researchers reported Thursday.

Their findings could be good news for what scientists predict about the risk of a killer flu pandemic, and they could also help researchers find better flu vaccines, the researchers said.

"Our work implies that we have never seen a true 'virgin soil' influenza pandemic," the team wrote in their report, published in the journal Science. "Virgin soil" means a population that has no immunity at all to a new infection.

The teams were looking at the puzzling characteristics of two strains of bird flu that keep popping up in people: the H5N1 and H7N9 avian influenza viruses.

They've been spilling over into people for years and scaring scientists who see the potential for a pandemic of flu that could kill tens or even hundreds of millions of people. But this hasn't happened yet.

And, oddly, while H5N1 has hit young people and children especially hard, H7N9 seems to attack older people.

H5N1 has infected 856 people since 2003 and killed 452 of them, WHO reports. H7N9 has infected 452 people and killed 124 of them, according to the Center for Infectious Disease Research and Policy at the University of Minnesota.

The team at the University of Arizona and the University of California Los Angeles dove into the numbers, examining the reports of each case, who was affected and how severely.

"It's ridiculously predictive," said Michael Worobey, an expert in viral genetics at the University of Arizona.

Worobey thinks he knows why. Other research suggests that early infections "imprint" the body's immune system. It makes sense — the human immune system makes cells and antibodies that "learn" to recognize and react more quickly to microbial invaders the next time they infect someone.

Worobey has also found that influenza viruses that commonly infect people can be categorized into two groups. Group 1 consists of subtypes H1, H2, and avian H5, while group 2 includes seasonal H3 and avian H7.

Influenza A viruses all are named for two proteins found on their surfaces: hemagglutinin (the H in a flu name) and neuraminidase (the N in a flu name). Thus flu strains get names such as H5N1 or H1N1 or H3N2. So human H1N1, H2N2 and the H5N1 bird flu virus are all related in Group 1, while the H3N2 seasonal flu virus is related to the H7N9 bird flu strain.

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