At 10:11 AM 8/17/03 -0700, Tim May wrote:

Many evolved diseases _DO_ kill their hosts. Look around.

It is true that there are tradeoffs in lethality, time to death, and virulence, and that a disease which kills too quickly and too many won't spread adequately, but quite clearly all of the diseases of the past were evolved (until recently, none were created) and yet they often killed their hosts.

This objection jammed in my memegrinder so I had to examine it. I'll argue that the nastiness of many human diseases are *temporary* exceptions to the "evolved pathogens don't kill" observation. Because humans are not in equilibrium: * Human population is growing. This means you can kill your host, two new ones are born every minute (except in a few places, eg W. Europe). If your host population is growing like that, you can be extra lethal, temporarily. If the host numbers are stable, you could wipe them all out if you're too lethal. * Humans are expanding their range. This means new diseases are introduced from existing resivoirs so they have not adapted to humans --especially the conditions of modern humans-- yet. Ebola, HIV, etc. * Humans only *recently* live in dense (and stationary) groups. This means that pathogens have not adapted yet. Cities are incubators. Bubonic plague, TB are good examples here. * Rapid travel is even more recent an invention. Populations who have never seen a pathogen (West nile, etc.) are getting exposed for the first time. No equilibrium there. The "Cortez" effect, amplified by Whittle's jet engine. "Globalization" means everyone gets exposed to everyone else's pathogens. A sick chinese chicken can ruin your day in America. Guns, germs, and steel. BTW Globalization also means that everyone gets exposed to everyone's plants, insects, etc. A lot of isolated species (e.g., Hawaii) that can't deal with competition will be toast just as much as the Amerinds who met Mr. Cortez. Guns, germs, and steel. Meet Mr. Kudzu. ..... Obviously, the scale of "temporary" should be taken in the larger context, not that of one's own lifespan. Of course a coadapted pathogen (eg flu) can spontaneously become newly virulent simply because of mutation or recombination. If the hosts aren't all connected, then merely one particular host-group dies, along with the newly virulent strain. Losing some village is not a big deal (until someone gets on a plane). .... Interesting to extend the analogy to say virii that zap cellphones or PCs permenantly vs. merely being annoyances. A PC-zapping virus would give Macs the kind of ripe open field not seen since the days of the Bering Strait. Also interesting to view the RIAA vs. Networked-Computer struggle in a biological ("evo/eco") light. Ms. Dodo, meet Mr. Kudzu. And of course fascinating to watch how the new dense mobile humans (or their lawyers :-) adapt behaviorally.