Re: [Marxism] Pig Flu: 'There Will Be No Pandemic' -- Susil Gupta

["Richard Levins" <humaneco@xxxxxxxxxxxxxxxx>]

I would like to add something to the discussion of the evolution of pathogens. 
Look at it from their point of view: a pathogen needs a good meal, safety from 
the body's defenses, and exit to another host. These three demands may coincide 
or conflict. Consider where the microbe hangs out: the blood is a great place 
to feed (good nutrition) but is very accessible to the immune system, and 
provides a good exit only with the help of blood sucking vectors such as 
mosquitoes. The central nervous system is also rich in nutrients, relatively 
protected, but with no exit. The skin is poor in nutrients, safe, and with easy 
exit. A pathogen may juggle these selective forces in various ways or it might 
make the blood safe by switching antigens every few days (trypanosomes) or by 
knocking out the immune system (HIV).
 Next, what is the relation between the fitness of the pathogen and the harm it 
causes? Consider a diarrhea. If the pathogen adheres tightly to the intestinal 
wall it can reproduce there and reach high density. Or it can produce more 
diarrheic symptoms and escape to look for a new host. How much time it has 
depends on how soon the body's own or medical responses make the place 
dangerous, and how easy it will be to find a new host. Here is where social 
differences (immune compromise etc) and the seed of medical intervention, and 
the durability in the environment it is expelled into, and contact with new 
hosts in that environment, all influence the evolution of the pathogen. The 
generalizations that pathogens evolve toward greater or lesser virulence are 
not valid because of the ver different and changing contexts for natural 
selection in an epidemic.
 

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Richard Levins
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