1. They moved to the cities to find new opportunities, and start new lives with mostly consistent paying jobs.
2. The urban rush led to over population and bad living environment. They would cram multiple families into one bedroom apartments.
3. European immigrants came to New York to find jobs and better lives. Millions also flocked because of the potato famine.
4. Bad sewer systems led to the spred of virus and bacteria. This also made disease's from other continents spread to those who had no immunity to said disease.