Factual error: Given the destruction from the nuclear blast threw the truck in the air, the rest of the surrounding area would also have experienced similar destruction. However when they are walking away from the truck to another town in a nearby field, there is no visible impact of what was a recent nuclear explosion anywhere to be seen.
Factual error: The movie is supposed to take place in Iowa in the spring according to the mayor who says they can't shut off the water because of farming, but says this while hanging out at his pool... in Iowa... in the spring...? BTW almost all farmers, and people in the country in general, have individual wells for water.
Chosen answer: Because he's naturally immune to the disease, just like all of the other survivors at that point.
Phixius ★