Epidemics usually start in areas of large population . Poor people in big cities who live crowded together in miserable conditions have the most health problems . They often have the least education about disease prevention . If they know what to do , they often do not have the money to do it . For example , it is difficult for a person who has no electricity to refrigerate food or boil drinking water . With no money , the person can’t even buy soap to wash his or her hands