Is there a way to say 'lack of appetite' in one word? (for instance: 'disappetite' which I found a very few results of it and I'm not sure how much is acceptable).
The patient had disappetite(?).
The medical term for reduced appetite is anorexia. This is the normal medical term for reduced desire to eat for a variety of causes, e.g. illness such as common cold, hormone imbalance, influenza, fever, and others. However, this generic term for appetite loss should not be confused with anorexia nervosa, which is a mental health disorder.