Rapid urbanisation is the process of people moving from rural areas to urban areas, which increases the size of the cities to which they move. While this has a number of positive effects, such as more jobs and other opportunities in a concentrated area, there is a host of problems with rapid urbanisation. This is especially true if the urbanisation is unplanned and allowed to spread unchecked.
Rapidly-urbanising areas often have more people than the roads can handle. This is because it takes much longer to plan and build an effective transportation system than it does to plan and build houses and commercial buildings. So, a rapidly-urbanising area will experience extremely heavy traffic as bottlenecks form from more people using a road than the road was designed for.
A rapidly-urbanising area will attract people from all over the surrounding countryside. The rapid influx of people creates a system where wages are depressed. This in turn encourages foreign companies to take advantage of these low wages by investing, which further-perpetuates the cycle of low-wage, low-skilled jobs. This makes it difficult for rapidly-urbanised cities to develop higher-level economies that generate more wealth.
It is difficult for infrastructure like water and trash collection to keep up with a large influx of people. This creates a situation where people are more likely to get sick, as they do not live in sanitary conditions. Medical services are also few and far between for the same reason, which compounds this problem. This combination of factors means that people who live in rapidly-urbanised areas have a tendency toward ill health.
Rapid urbanisation means that people move further from the city centre. This creates a situation where people -- and resources -- are not evenly distributed. Schools, for example, can become extremely crowded in the suburbs on the periphery but empty and falling to pieces in the city centre. Sprawl also adds to traffic and other infrastructural problems when people live in the suburbs but work in the city centre, as commutes are both long and crowded.