What brought about the end of the Yellow Fever epidemic in Fever, 1793 and why?

The cold weather brought about the end of the yellow fever epidemic. Yellow fever is spread by mosquitoes; people continued to get sick as long as mosquitoes proliferated. Mosquitoes thrive in hot, humid weather. It was only with the coming of the first frost that the mosquitoes were killed off and the epidemic came to a close.

The yellow fever epidemic that struck Philadelphia in 1793 began in August and ended almost four months later in November. The weather during that time of the year in that part of the country is generally oppressively humid and warm.  Although much less was known about yellow fever during the eighteenth century, most people understood that the fever would end with the coming of the first frost. It was thought that the cold, clean air killed the germs that gave rise to the fever; the connection of the cold weather with the spread of the disease through mosquitoes was not yet understood. Be that as it may, the change in the weather brought a clear sense of hope and relief to the people of Philadelphia, and, in fact, the disease did diminish correspondingly. Doctor Benjamin Rush writes in a letter, "Blessed be God for the change in the weather. The disease visibly and universally declines," and when Matilda awakens in Chapter 26 to see a "white veil that lay over the weeds," she exclaims to Eliza, "It's frost! The first frost! The end of the fever!" Sharing her sense of wonder and gratitude, Eliza responds, "Lord have mercy...we made it."