Constipation is a common Betta fish disease because they are easy to overfeed. In fact, most Betta feed containers recommend an amount of food that is 1.5 to 2 times what your fish actually needs each day. If you fear your Betta has overeaten and is constipated, for the next 2-3 feeding sessions microwave a frozen pea and cut out little pieces of the inner pea to feed him to help get things moving.Dropsy is a commonly known red flag for Betta fish infection and usually signals bacteria problems. Your fish may be bloated or, in severe cases, have scales that are sticking out from its body instead of being tucked flush like normal.
Fungus is a relatively common and usually not deadly affliction, but should still be treated. This is usually signaled by white fuzz or growths on your Betta's body and fins.
Fin Rot has stolen the luster from many a pet Betta fish and should be monitored closely and treated should it arise. Your Betta fins will become tattered and have holes in them as they deteriorate.
Parasites come in many shapes and forms and therefore cause different systems but as a rule of thumb excessive itching/rubbing against tank objects or trouble breathing are often signs of some sort or parasite problem.
Ich is a Betta fish disease that all Betta fish owners will probably read about at some point but (luckily) not all will have to deal with. However, Ich causes small white spots on the body and fins and requires a waterborne treatment.
Swim Bladder Disease manifests itself in buoyancy problems that cause your fish to swim sideways or in some other abnormal manner.
Septicimia is one of the serious Betta fish diseases that should be treated as such and is usually identifiable by a number of symptoms converging at once. This often includes a refusal to eat food for a long period of time, extreme lethargy or lack of energy, and/or red streaks on your Betta fish's body.
Tuberculosis is the only Betta fish disease I know of that is transferable to human beings and is usually signaled by a dulling in color, popeye (another lesser Betta disorder where you're fish's eyes protrude more than normal), and fins that are used little and kept clamped close to the body. It is worth noting, however, that many things (including just aging sometimes) can cause a Betta's colors to dull, so your pet does not necessarily have tuberculosis if you notice a lackluster appearance but the fish appears to be fine on all other counts.
Velvet is a relatively common but treatable Betta condition. However, early detection is key to survival and the most notable signs are, as with some of the diseases above, clamped fins and lethargy, along with weight loss. However, the distinctive symptom of Velvet is a fine layer of rusty aspect to the Betta's skin which is most easily visible in the beam of a flashlight or something similar.