Yellow fever virus is found in tropical and subtropical areas in South America and Africa. It's extremely rare in North America.
The entire community is threatened by the "highly contagious" disease of Yellow Fever. In fact, Yellow Fever is transmitted by the bite of an infected mosquito - it is not "highly contagious" from human to human at all.
In the scene in Hattie's General Store, a Dutch Masters cigar box is clearly shown on a counter. Its distinctive packaging features Rembrandt's 1662 painting The Syndics of the Drapers' Guild (De Staalmeesters), which is shown on the side of the box. Dutch Masters cigars were not sold in the US until 1911.
When Lucas is sending Mark off with the Doctor (same scene that Mark calls him Dad), you can see an automobile on a highway in the background.