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Message: <blockquote data-quote="Thomas Shey" data-source="post: 8197057" data-attributes="member: 7026617">That's the gig here.  Mutation, essentially, rolls the dice for new traits, and then selection picks the versions of an organism that does best, reproductively speaking out of them.  Virii and other microorganisms have rapid life cycles, so you get to see the process in realtime in a way that a lot of other organisms do that too slowly to do.All other things being equal, host mortality increases are counterselected for.  That's true even of things like AIDS; a host that lived out its whole life as a virus generating factory would be more beneficial to reproduction of the organism than one that doesn't.  But once the survival time is long enough relative to the virus life-cycle, that selection pressure is minor, whereas other elements may be stronger.That's why really quick-kill diseases are rare; they can take down the host before it even gets a chance to spread the disease, and the counter-selection there is stronger the shorter the time frame is.  The longer it takes, the more other reproductively benign traits start to weigh in more than that.But this still tells you why "super deadly and super quick diseases" aren't really common; its a poor reproduction strategy, and that quickly selects away from it, and the selection away from deadly diseases never really stops. Its just the effect on slower-killing diseases can be kind of weak.</blockquote>