Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Sialadenitis ?
    ### Response:
    Sialadenitis is an infection of the salivary glands. It is usually caused by a virus or bacteria. The parotid (in front of the ear) and submandibular (under the chin) glands are most commonly affected. Sialadenitis may be associated with pain, tenderness, redness, and gradual, localized swelling of the affected area.
    