negative effects of more and more people from rural ares are blocking to big cities?

I believe that this question is asking a general question about possible negative effects of people "flocking" to cities. I'll list out a few possible negatives.
Disease. Cities offer large numbers of people in fairly tight and contained spaces. People share common areas, bathrooms, tables for eating, etc. Various diseases are transmitted in various ways; however, host bodies (people) that are in close proximity to each other make it easier for pathogens to move from person to person. Cities can be breeding grounds of poor health.
Infrastructure stress. If a city is currently built to adequately house or employ 500,000 people, and then people flocking to that city drives up the population, the city's infrastructure might not be able to cope and change as rapidly as the population. Traffic congestion would be an issue as well as too much demand on the city's utilities.
Unemployment. This goes along with number 2. People can absolutely flock to a city; however, more people might be coming in than there are jobs available.