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They discovered that people born before the Hong Kong flu pandemic of 1968 – in which H7N9’s ancestor, H3N2, first emerged – were largely protected from the older-type H5N1, but were vulnerable to severe illness from H7N9. Those born after 1968 showed the opposite responses. This explains a pattern that has puzzled scientists since 2013, when H7N9 itself was first detected: flu viruses from the H5N1 “family” disproportionately affect children and young adults, while those more closely related to H7N9 are more likely to infect older adults. - cosmosmagazine.com