If its an outside cat it can be many possibilities. It could have gotten in a fight with another animal or even fell and hurts itself. If its an inside cat there are many possibilities as well. All in all, if the cat seems to be moaning or seems very hurt you should bring it to the veterinarian as soon as possible.
This could be a variety of things. Flea bites can cause nasty sores if the infestion is particularly bad, or if your cat has an allergy to fleas. It could also be some other parasite, allergy (food can cause flaky and itchy skin if it has been changed recently or it could be some other external factor) or some kind of dermatitis. It is advisable to take your cat to a veterinarian as soon as possible as they will be able to diagnose the problem.
The cat very likely has a skin condition or irritation. The most common cause of scratching, biting, and fur loss is----FLEAS! Treat the cat with flea med and anti worm medicine. If the skin doesn't start to heal, see a vet.
The cat could be allergic to fleas. Take him to the vet to get him checked out, and treated for fleas.
It may be caused by too much scratching from fleas.
Puss is a reaction to an infection. If your cat has sores on its back paws that are leaking puss, your cat needs to be seen by a veterinarian.
Sores on your back can be caused by bacteria or disease. Impetigo can cause these, as can a problem with acne that has spread to the back.
No take him to the vet its ringworm.
Fougera vitamin A and D ointment can be used on a cat's skin to treat sores. There is no known danger or health risk associated with external use.
This could be a type of mange. You should bring the cat to the vet because mange is not only lethal but certain types are extremely contagious.
Just like with your face, you can get pimples on your back if pores get plugged up. It is also possible to get sores from poor hygiene.
Breast Milk
Sores behind your ears can be acne. You can make an appointment with the dermatologist to have him or her look at the sores.
Many things from Acne to rubbing of a wetsuit
yes
impetigo
No, you have sold you cat and it is no longer yours. Unless whoever you sold it to offers it back (you will probably have to give them back their money if this happens) then you can not get your cat back.