Just another comment on purpose

Ian A. York iayork at panix.com
Mon Sep 16 09:47:37 EST 1996


In article <51cp4h$pnk at net.bio.net>,
Martin Hewlett  <hewlett at brahms.biosci.arizona.edu> wrote:
>
>I think that this is the key point of the "balance" argument.  It seems
>to me that a successfull virus must somehow allow for the presence of
>"further hosts" in order to propagate.  Therefore, it is difficult to
>see how case fatality rates approaching 100% are consistent with this
>need.

Not so.  First, as far as many non-fatal viruses are concerned, a host is
a one-shot case anyway (i.e. after a host is infected, it becomes immune,
and the virus's progeny have no practical chance of re-infecting it; that
individual host is out of the pool of susceptible individuals).  Since
that host is of no more use to the virus, it cares not whether she lives
or dies - it can't even detect the difference between a dead host and an
immmune host.  (Sorry for the anthropomorphizing.  You know what I
mean.)

If the host dies, it leaves no more progeny; this might present a virus
with a problem in the long run, but that's not necessarily the case.
Hosts die anyway; individuals produce a more-than-replacement number of
progeny.  The key is that the virus maintains a roughly steady level
within the population over time.  A population can replace the ravages of
a 100%-fatal virus.

Analogy time: mice are preyed upon by owls.  Damn few mice survive an owl
attack - owls are close to 100% fatal for mice.  But there is no selection
pressure on owls to become less lethal on an individual basis; there is
pressure on the owl population as a whole to not become more abundant
than the mouse population can support - if they do, the owls starve to
death, the mouse population recovers, and the status quo is restored.

The situation is exactly the same for viruses.  If a virus is 100% fatal,
and it becomes too abundant, then each individual it infects is less
likely to encounter (and re-infect) a new host; the virus becomes less
abundant, the population recovers; now infected individulas are more
likely to infect new hosts, the virus bounces back, and so forth.

One strategy for the virus is to allow the host to survive longer, and
that may let it encounter more potential new hosts.  But there are many
other also potentially successful strategies: it could survive longer
outside the host, it could become more infectious (enhance the chance of
infecting those new hosts it does encounter), it could cause the host to
run around more to enhance the chance of meeting new hosts, and so on.

Rabies is close to, if not at, 100% fatal.  It's a highly successful
virus, and its strategy hasn't changed over the centuries.  Rabid animals
run around a lot, increasing their chances of meeting new hosts; they
tend to be exciteable and snap at things, enhancing the infection of the
virus in the salivary glands; and so on.

You're exactly right to say that the key point is one of balance.  You're
exactly wrong to assume that there is only one way of balancing things,
and that that way is to become less lethal.

Ian
--
      Ian York   (iayork at panix.com)  <http://www.panix.com/~iayork/>
      "-but as he was a York, I am rather inclined to suppose him a
       very respectable Man." -Jane Austen, The History of England





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