Infectious lymphadenitis: This is often due to a viral or bacterial infection, such as a cold, flu, or local skin infection. The location behind the ear suggests an infection in the scalp or face that has spread to the lymph nodes.

Dermatological conditions: Conditions like eczema, psoriasis, or contact dermatitis can cause lymphadenopathy due to the body's immune response to skin irritation or inflammation.
Ear infection: Otitis externa or media can lead to lymphadenitis in the lymph nodes behind the ear due to the spread of infection.
Dental infections: Infections from the teeth or gums can drain to the lymph nodes in the neck, including those behind the ear.

Lymphoma: Although less common, lymphoma can present with lymphadenopathy in any location, including behind the ear. It's crucial to consider this diagnosis to ensure timely treatment.
Metastatic cancer: Certain cancers, like squamous cell carcinoma of the skin or other head and neck cancers, can metastasize to lymph nodes behind the ear.
Cat-scratch disease: Caused by Bartonella henselae, this infection can lead to lymphadenitis, typically after a cat scratch or bite.

Kawasaki disease: A rare condition that mainly affects children, causing inflammation in the walls of medium-sized arteries throughout the body, and can present with lymphadenopathy.
Sarcoidosis: A disease characterized by the growth of tiny collections of inflammatory cells (granulomas) in any part of the body, including the lymph nodes behind the ear.
Tuberculosis: Although more commonly associated with pulmonary symptoms, tuberculosis can cause lymphadenitis in any location, including behind the ear, especially in immunocompromised individuals or those from endemic areas.