How would crowded conditions in cities favor spread of plague?

Person to person contact and waste systems.

Explanation:

In crowded conditions humans are closer together and tend to have more physical interaction, so for airborne viruses, it would be easier to spread from one person to another, also applies for viruses that spread through fluids, more contact with infected people means more possibilities of propagation. Another way in which cities can create a good environment for plagues is due to contamination if it is a bacterial or fungal virus it could reproduce easier in landfills, and drainage systems in cities since they carry far more waste in cities because of a higher density of population.