The most likely cause is fungal skin disease, which is common cat ringworm. At this time, the cat’s bald area will have a small amount of dander or scabs, and the owner needs to clean it and apply fungal drugs for treatment. It takes a period of time for new hair to grow; bugs outside the cat's body can also cause baldness. In this case, the cat will feel very itchy and needs to be dewormed internally and externally by the owner after bathing it.