What are some of the causes of oral thrush in adults?

See answer. Excess use of antibiotics, diseases that suppresses your immune system like diabetes, hiv, lupus, leukemia and drugs that suppresses your immune system like steroids and transplant drugs.
DOWN= Immune system. Immune deficiency has to be suspected if thrush is diagnosed. 'immuno-compromise' can be due to a virus (hiv, hepc etc.) or due to medications that suppress the immune system e.g. Steroids, meds for rheumatological disorders etc. Also recent antibiotics can alter 'the bacterial flora' of the body and may cause 'yeast infections' such vaginal yeast or thrush. The treatment is topical 'anti-fungals.