If people used it more often this book would fall into New Adult for me. It could be considered a very heavy book for the younger end of YA, but people used to the new horror movies and such would probably find this book just fine. It is also firmly in the sci-fi genre for me despite being labeled as horror on goodreads because frankly there is no horror. The book also spends a decent amount of time discussing the science behind these "hungries" and how to get a cure. 
Overall it was a middle of the road book for me. It was neither great nor terrible. I didn't personally find it super disturbing or anything, but I also don't tend to find anything too too terrible.