Hip
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Or has *something* about modern living (say the past 100 years or so) caused us to become more susceptible?
Two factors in modernity that likely plays a role in how pathogens evolve and mutate is urbanization and transportation: as we increasingly urbanize, human beings begin to live in closer and closer contact with each other, all of us spreading germs to each other (in offices, crowded public transport, shops, busy bars and nightclubs, etc), and modern transportation — ie, cars, trains and aeroplanes — allows pathogens to travel easily from one city or country to another.
Evolutionary biologist Paul Ewald argues that whereas infectious pathogens in rural environment (like much of Africa) tend to evolve to become more virulent and more lethal, in urban environments they do the opposite, and become less virulent and less lethal.
This is not because infectious pathogens are developing compassion and a soft spot for humanity, but simply because in crowded populated environments like cities, the pathogen's chances of transmission to a new host (another human) are greater if the pathogen does don't rapidly kill the human, because then that human can keep circulating among other humans, and the changes of transmission to lots of other humans is much higher.
So for this reason, in crowded urban environments, pathogens will tend to evolve into more benign, less virulent and less lethal from, argues Paul Ewald.
In primitive rural environments, where people live in villages of just a few hundred people, and where transport links are sparse and very slow, a pathogen does not have much opportunity to infect millions of people. So there is no advantage for the pathogen in keeping its host alive, because that infected person does not circulate among millions of people, only the few people of the village. Thus in rural environments, there are very few opportunities for the pathogen to jump into a new host.
Thus in order to maximize its chances of infecting as many other rural villagers as possible, the pathogen may evolve into a nasty virulent form, perhaps causing lots of nasty infected sores on the skin of the host which ooze out infected puss, and then killing the host so that the other villagers might be infected by the sores as they bury him.
Creating a nasty, bloody, puss-ridden rapid death is an excellent strategy for pathogen in a rural environment to maximize its chances of transmission.
So what are the consequences of pathogens evolving into less virulent forms in urban environments? Perhaps pathogen-associated chronic diseases like ME/CFS are the consequence. If pathogens no longer want to kill us outright, but prefer to keep us alive so that we can spread the pathogen to others in the city, we might see an increase in pathogen-associated chronic diseases. We can have a chronic disease, but as long as we are healthy enough to continue circulating among other humans, coughing and spluttering our germs over them, that benefits the pathogen.
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