Letting the fire go out increases the likelihood of them getting sick. It's best, if you're going to be away for awhile to add more wood to the fire, and slow the speed down.

Thanks for quick response, I thought it would do something. I usually always restock the fire & put it on slow if away most of the day, normal for over night. If I know i'll be away for a long period I put it on pause. Thanks again for letting me know what can happen.

However, if you have a small army of doctors, the fire burning out shouldn't be a problem.