HHN is truly a beast of its own when it comes to staffing. There is always a Health Services TM at the exit of every house. In addition, there are many that roam the park. If you're not near a haunted house/show, find any TM, preferably one with a radio, and they will get Health Services to you.
In the case of medical emergencies, ambulances can be called. Health services, security & OPD are everywhere, so you can always find someone to help.