I can tell a tale about infections in dense settlements and stupid Bannis first passing hospitals to go to a marketplace, in school or to another well-visited site, infecting everyone there and everyone he meets on his way. I wish that not everyone gets the Yellow fever in your settlement.
By the way; I think that the sick person visits the nearest hospital to his home. In these large settlements, the "reassign jobs" function doesn´t work (at least that´s what I´ve seen and I´ve never been close to your 8000 inhabitants) and more people are on the roads than usual.
One question; why do you import different kinds of fruit? If you only bought apples, your problems would be out of the way.