It doesn't, which might seem counterintuitive. I guess that's why they call it the paradox of tolerance instead of the obvious fact of tolerance.
Oh, I
love obvious facts. There's nothing better than a set of self-evident truths so patently obvious to all as to make the very idea of dissent ridiculous to get people to do something really stupid
en masse. At any rate, given how adept various fringe groups have gotten at recruiting people by claiming they're being officially suppressed and censored, perhaps we should be less ready to resort to officially suppressing and censoring them as a means of expressing our intolerance of their intolerance.
Also, I feel it is less about being filthy and more about being quite densely packed full of people and animals, and open meat markets and horrible things in the rivers, due to lack of knowledge about what those things could bring with them... personal hygiene can only help so much when the whole city is full of rats, and pets, and so crowded with people close together with no knowledge of things like germ theory, that if one person gets it, in two days they could infect a huge number of people.. I feel as though the main reason we aren't so plagued by plagues is that we have things like anti-bacterial stuff and germ theory...
And you'd be right, at least in the sense that zoonosis and cities are a heck of a combination for making really fatal epidemics. (there's also some questionable population genetics being put forward here, but that's a separate text wall.)