What was the main reason disease spread rapidly in american cities in the late 1800s?

There were many reasons why disease spread rapidly in American cities at the time, but I believe the most important one was overcrowding.
There were a lot of people living at the same location, but unlike today, they didn't have proper sanitation and living conditions, which was why various diseases could spread freely and contaminate food and people.

In my honest opinion , i’d say franklin d roosevelt was a president that impacted our nation , i say this because fdr is voted among top three best us presidents and he was the longest serving president by 3 terms , usually presidents only supposed to have two terms increments of 4 years , equivalent to 8 years total before election time. and he established the fdic , through the 1933 banking act and he established the us , social security system , during his presidency, and established a national minimum wage , through the fair labor standards act , fdr led the allied coalition to victory in wwii and he played a major role in the creation of the united nations.