<p>Very interesting question! I'm not sure what the answer is but I remember reading about the 1918 flu pandemic and how it was so unusual in that it was so much more dangerous for young adults than it was for children or the elderly. Obviously chickenpox/measles are not the same thing as the flu but I still thought this was an interesting explanation as to why they think it happened that way</p>
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<p>"Most influenza outbreaks disproportionately kill juvenile, elderly, or already weakened patients; in contrast the 1918 pandemic killed predominantly previously healthy young adults. Modern research, using virus taken from the bodies of frozen victims, has concluded that the virus kills through a <a href="http://en.wikipedia.org/wiki/Cytokine_storm" style="background-image:none;color:rgb(11,0,128);" target="_blank" title="Cytokine storm">cytokine storm</a> (overreaction of the body's <a href="http://en.wikipedia.org/wiki/Immune_system" style="background-image:none;color:rgb(11,0,128);" target="_blank" title="Immune system">immune system</a>). The strong immune reactions of young adults ravaged the body, whereas the weaker immune systems of children and middle-aged adults resulted in fewer deaths among those groups.<sup id="user_cite_ref-Barry_book_11-0"><a href="http://en.wikipedia.org/wiki/1918_flu_pandemic#cite_note-Barry_book-11" style="background-image:none;color:rgb(11,0,128);white-space:nowrap;" target="_blank">[11]</a></sup></p>
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<p><a href="http://en.wikipedia.org/wiki/1918_flu_pandemic" target="_blank">http://en.wikipedia.org/wiki/1918_flu_pandemic</a></p>
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<p>Just food for thought...</p>