In Fever 1793 by Laurie Halse Anderson, what were signs that Grandfather had yellow fever?

The first
sign that Grandfather could be ill from yellow fever is in chapter 11, page 78, when he begins
coughing in the back of the carriage on the way out of Philadelphia. Initially, he waves off the
coughing fit as nothing, but on page 81, they are stopped by guards making sure no one with
yellow fever...