Two flu subtypes have circulated seasonally since 1977, with very different effects on different age groups.

Now, a new study in the journal PLoS Pathogens suggests a reason why.

For decades, the H3N2 flu subtype has accounted for most deaths and high-risk cases among the elderly, while H1N1 has more strongly affected young and middle-aged adults.

Researchers found the difference was not due to how quickly the subtypes evolved. Rather, our immune systems develop a lifelong bias toward resisting the first flu subtype we encounter as children.

"There's something about that very first exposure that gives you the very best possible immunity for the rest of your life and interferes with your ability to create as good an immune response to the other subtype," said co-author Michael Worobey, head of the University of Arizona Department of Ecology and Evolutionary Biology.