Early on in Fever, 1793, rumors spread that it's the refugees who are responsible for the fever in Philadelphia. While many fleeing from Santo Domingo (now Haiti) were indeed carrying the virus, the way in which the topic gets discussed becomes a bit of a problem. The fever is something that "they" are causing. Not "us." In this case, the "other" becomes the scapegoat for a problem infecting an entire society. As we learn from the novel, this kind of "us" versus "them" attitude can lead to acts of dehumanization (that is, treating people as though they aren't human beings.) The sad truth is, though, that death, fever, and illness can happen to anyone – refugee or otherwise.
We should also note that there are different kinds of foreigners in the novel, and each are treated in different ways. The French Doctors, for example, present the right cures, but they're in direct conflict with the treatments prescribed by the American Dr. Benjamin Rush. There's also the Free African Society that Eliza is a member of, and that helps victims of the fever and their families.
By viewing people as the "other," they become dehumanized. It becomes easier to dismiss people or treat them poorly when we regard them as less than human.
Philadelphia is a city of many cultures, all of them valuable to the life of the city.