Viral infections - the most common one that causes mouth sores, drooling, and foul breath is a serious infection called calici. This can cause ulcers
on the gums, tongue
, lips, roof of the mouth, throat, and lungs. This is one of the two most dangerous viral upper respiratory infections. It commonly leads to bacterial infections that require antibiotics to cure. Pneumonia is also a somewhat common complication. Most cats will stop eating when suffering this infection due to mouth pain.