What do you mean and how do you know it? I’m guessing that you mean that density causes exposure to microorganisms adapted to humans. But we’re probably a lot less exposed to random microorganisms from eating dirt (though it’s not clear they should be called “pathogens”).
The hygiene hypothesis of allergy is speculative, but here’s a concrete, widely accepted claim: before 1900, infants contracted polio and had very mild cases. After 1900, they contracted it at a later age and had bad cases. This suggests increased hygiene at the same time of increasing density, apart from the question of whether hygiene is good or bad at the current margin.
What do you mean and how do you know it? I’m guessing that you mean that density causes exposure to microorganisms adapted to humans. But we’re probably a lot less exposed to random microorganisms from eating dirt (though it’s not clear they should be called “pathogens”).
The hygiene hypothesis of allergy is speculative, but here’s a concrete, widely accepted claim: before 1900, infants contracted polio and had very mild cases. After 1900, they contracted it at a later age and had bad cases. This suggests increased hygiene at the same time of increasing density, apart from the question of whether hygiene is good or bad at the current margin.
Yes population density, international travel combined with the huge human population, and also the way do and have raised farm animals.