Theres quite a few diseases for reptiles, and I will be going over a few of them. Take your reptile to a reptile specialist vet if suspected of anything.
MBD - This happens when you dont give enough calcium, dont have a good UVB bulb, or the uvb bulb has stopped producing uvb, it can never be fully cured but at least you can try and give a bearded dragon like this a better life by taking better care of it. A few symptoms of it is dragging the back legs, twitching of legs/feet/hands, underbite, paralyzation, shaking, tremors, and stunted growth.
Impaction - Occurs when something is lodged in the digestive track blocking everything up. This can be deadly if not treated.
Usually particle substrates like calci-sand, vita-sand, any sand, repti-bark, gravel, ground walnut shells, etc. This can also happen if the prey item is too big, or the temperatures are not high enough to digest the food.
Symptoms of it include lethargy, dragging of the back legs, and not having any bowel movements (going to the bathroom).
Tail rot - Causes are usually too low of humidity levels (resulting in stuck shed multple times) or tail trauma (getting stuck, bitten etc). Usually looks very black, dead, or shriveled. If not treated it can work its way up to the body infecting the organs and causing the beardie to die.
Mites - Nasty external parasites that feed on your bearded dragons blood. Too many mites can kill them, they are hard to get rid of and reproduce like crazy. They tend to be transported from one reptile to another. The first thing you want to do when you have mites is make sure the animal is in absolute quarantine. Don't even let things it has touched near the other animals.
Internal parasites - Symptoms are lethargy, not eating or pooping in their normal schedule, and extremely runny and smelly feces. Usually pretty easy to get rid for some with medicine, other internal parasites may be harder.
RI - A respiratory tract infection is when there is a bacterial infection in the lungs. Caused by too cold of temperatures, too high of humidity, and sometimes stress. Symptoms of Respiratory Infections include gaping constantly, mucus coming out of the mouth or nostrils, and a clicking or rattling sound when breathing.
Yellow Fungus - It is a contagious disease. It is thought to be caused by yeast infections in the digestive system. It is usually found by the appearance of yellow patches of fungus on your Bearded Dragon. A vet is a absolute must.