Interesting - this (http://www.nytimes.com/2009/06/24/health/research/24fat.html...) recent NYTimes article reported on a new hypothesis that obesity may have offered an evolutionary advantage against tuberculosis, by "revving up" the immune system. I'm not sure about swine flu, but I recall that avian flu kills by forcing the immune system into overdrive (http://en.wikipedia.org/wiki/Cytokine_storm), killing the host. Is this a possible mechanism? Anyone know more about influenza or the immune system?