How might the setting from chapters 1-5 speed up the spread of yellow fever?

The close quarters and lack of hygiene in the city of Philadelphia add to the contagion and rampant spread of yellow fever. People talked of the illness, and the mounting deaths were counted and tolled, but the people were unaware of how to keep the contagion in check, and those who were infected often moved throughout the city until they dropped. There were also those who fled the city, taking the fever with them.