I'm not sure I see this as horror per se. To me horror means monsters and the supernatural. Hill's Heart-shaped box fits the genre but I'm not sure about this one.
This is about a very real scenario where a fungus running rampant in the US can cause spontaneous combustion in the host. As the infection spreads, fear spreads also. There's an all-out war to simply eliminate all who are carriers. Especially in this era when fear of your neighbors seems de rigueur it's chilling. School nurse Harper joins up with some people who seem to have found a way to survive in harmony with the fungus. Unfortunately, sometimes this can mean discordant harmony. Harper finds herself in danger and flees in the company of a few like-minded carriers.
Reading Joe Hill is much like reading Stephen King. A book like Under the dome could easily have been written by Hill. For me the books are good but I could do with a little less violence and/or a little less graphic violence.