Firstly, you have to check the entire animal to see if there are fleas and if you find any, see how to beat them in the note below.

If after beating them, the cat still loses hair and/or has irritation, you have to visit a veterinarian.

In case you do not find any fleas, the animal has to be checked by a veterinarian to determine what kind of pathology is affecting it and what type of treatment is needed to heal it.