What are the causes of emigration and what are the problems associated with it?

Immigration is mostly caused by the lack of products and clean water in foreign countries. For example, Mexico has horrible living conditions, and America has it much better, so they want to come over here. The problems associated with it are the ones who come over here to sell drugs, kill people, etc. Another problem with immigration is that to be in America you must have a Visa or a Green card, and most people who come over don't want to get one, or can't.