Lack of sanitation allowed for standing pools of water to form, which let mosquitoes breed, which led to the yellow fever spreading. So, I think the yellow fever epidemic was started by lack of cleanliness.
I believe the epidemic of yellow fever started because lack of sanitation and poor medical care. People did not know correct sanitation issues and they had not yet discovered germs and how they spread. The doctors were not so great (don't get me wrong sometimes they helped). Doctors did not have right tools and medicine to cure patients.
