This wouldn't be such a big issue except for the incredible affinity humans have for living in their own filth. People build bigger and bigger cities, with more and more people wandering around soaking in the byproducts of their own emissions, breathing each other's air, drinking treated sewage, etc etc etc. Apparently we like living on top of each other enough to offset the minor downsides like crime, chaos, filth, crowding, and early deaths.
Humans: Termites with hands