Does your cat scratch a lot? Or your cats over-groom for more than hygiene which can result in injury from excessive licking, biting, and results bald patches or redness to the skin. Have your vet check for dermatological problems or parasites first, as all these need to be treated. Skin infection (fungus), lice, fleas and ticks can be a reason.
Skin infection (fungus), Usually if you bring them to the vet, the vet will examine and screening you cat using UV light. From UV, you can see the a glowing green spot (protein) at the skin which indicate the fungus existence. Some medicated shampoo routine will be advised and medicated cream or solution to apply to treat fungus. I use Malaseb Shampoo and NaturalPet antifungal lotion.
Flea, ticks and lice:
You can check your cat at home by yourself. Screening over the furs and you can find a dirt like black dots or reds one or “moving” dandruff between the skin. For white coat cat, it may easy to see but for colored fur, you may take a damp white cloth and wipe on you cat coat. You may see the black or red dot. That is your enemies. I’m using Frontliner Spray and Spot on. It’s promising and effective. 
Controlling an Outbreak:The eggs or spores can be transferred from a pet to anything in your house like carpets and household furniture. It is recommended which you also treat the home and check out other animals. Vacuum cleaning, disinfect the house and cages will do. I use Petdiatric Black Series to disinfect. Bear in your mind, there could be an issue of excessive stress. If your cat is grooming too much because of stress, such as a change in routine or a new animal in the house, you can take steps to ease the problem.
Good Luck!
Good Luck!
