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

[Richard Levins]

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