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0:00
The problem of the
evolution of virulence
is essentially the question, why
do infectious agents harm us?
If their survival
depends on our survival,
why have they not
evolved to be harmless?
What I'm going to do is illustrate
one way evolutionary biologists
attempt to answer this question,
and I'm going to do so
using malaria as an example.
I hope to persuade you that it is
both intellectually interesting
and important to know why malaria
parasites-- and by analogy
other parasites-- harm us.
I have prepared these slides, and
the opinions expressed are mine.
However, my presentation
draws heavily
on work of members of my research
group and other collaborators.
In particular, my view
of malaria evolution
has been heavily shaped
by long and very fruitful
collaboration with
Margaret Mackinnon.
0:42
Evolutionary biologists
often try to understand
how natural selection acts.
Here I will ask how
natural selection
acts on malaria virulence.
But before discussing
the natural selection,
I need to define virulence.
The word is used to mean a
variety of different things
by different people.
I use it to mean the harm done
to us following infection.
In other words, the
things physicians
worry about, morbidity
and mortality.
Other things, like a parasite's
ability to infect or replicate
or transmit, are related
and very important,
but they are not part of
my definition of virulence.
I'm going to ask how
natural selection shapes
the virulence of malaria
parasites in two steps.
First, I'm going to ask,
why are they so virulent?
What evolutionary
advantages are there
for parasites which
harm their hosts?
Why is malaria nasty?
It turns out that that is
relatively easy to answer.
In fact, it's so easy to answer that
it begs the next question, which
is, why aren't malaria
parasites more virulent?
Why aren't they nastier?
And I'm going to spend most of the
lecture on those two questions.
I then want to spend a few minutes
discussing the implications
of evolutionary
analysis for medicine.
I want to persuade you that
we cannot ignore parasite
evolution in public health planning.
This is obvious in the
context of drug resistance.
I hope to persuade you that
it is very likely to be
important for virulence as
well, and we are currently
overlooking this,
possibly to our peril.
And then I will end by very briefly
discussing two other diseases
to illustrate a precautionary plea.
