"Tonsilitis" is not a particular illness. It just means an infection of the tonsils. "-itis" means that whatever precedes it is poorly. So, "dermatitis" is a rash. "Hepatitis" is when the liver is ill, either due to one of many viruses or chemical onslaught (excessive alcohol, usually).
Anything that infects the throat can cause tonsillitis. You can get that from a client or anything.