An analysis of tissue samples from patients who died during the 2009 influenza A(H1N1) pandemic or the 1957 Hong Kong flu pandemic suggests that pandemic flu most often affects young, healthy individuals because previous infection with other flu strains fostered a harmful immune response (Monsalvo AC et al. Nat Med. doi:10.1038/nm.2262 [published online ahead December 5, 2010]).
Hampton T. Flu Clues. JAMA. 2011;305(5):458. doi:10.1001/jama.2011.55