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."