This double burden is often present in areas that have experienced rapid urbanization. Throughout most of human history, populations were not large enough to sustain highly transmissible infectious diseases for long periods of time. Now, however, this is no longer the case. Because people are living closer to one another in often unsanitary environments, the potential for infectious disease transmission is much higher. In addition to higher rates of infectious diseases, rapid urbanization has led to poor living and working conditions, and thus more chronic diseases. For example, poor urban individuals who live in moldy apartments are more likely to be afflicted with asthma. Furthermore, overworked factory employees are more likely to suffer from work-related injuries and environmental pollution.