List three factors that led to the spread of disease in cities

Three factors that led to the spread of a disease in the city is, the idea of bringing people without vaccination, the bringing of animals that come with diseases and the fact of visiting other places with diseases 

U-Uh W-W-Well, There Is Sneezing. No, the three factors are depending on the disease airborne, so much as a little poke, or through when a mom and a dad love eachother very much. I do not know how to answer this but this is my best

The answer is forindependence.