Mumps: This is the most common cause of parotid swelling accompanied by fever, especially in children and young adults who have not been vaccinated. Mumps is a viral infection that primarily affects the parotid glands, leading to swelling and tenderness, often accompanied by fever, headache, and malaise.

Parotitis due to bacterial infection: Bacterial infections, such as Staphylococcus aureus or Streptococcus, can cause parotid gland inflammation (parotitis), leading to swelling and fever. This condition is more common in older adults or those with compromised immune systems.
Viral parotitis (other than mumps): Other viruses, like influenza, parainfluenza, or enteroviruses, can cause parotid gland inflammation, although this is less common than mumps.
Sialadenitis: Inflammation of the salivary glands, which can be caused by viral or bacterial infections, can present with fever and parotid swelling.

Sjögren's syndrome: An autoimmune disorder that can cause inflammation of the parotid glands, among other symptoms. While less common, missing this diagnosis could lead to delayed treatment and significant morbidity.
Tuberculosis: Although rare, tuberculosis can affect the parotid gland, causing swelling and fever. Given its potential severity and the importance of early treatment, it's crucial not to miss this diagnosis.
Lymphoma or other malignancies: While rare, parotid swelling can be a presenting symptom of lymphoma or other cancers affecting the parotid gland or surrounding lymph nodes. Early detection is critical for effective treatment.

HIV-associated parotid cysts: In individuals with HIV, parotid cysts can occur, sometimes accompanied by fever and swelling.
Heerfordt's syndrome: A form of sarcoidosis that affects the parotid gland, causing swelling, fever, and sometimes uveitis or facial palsy.
Parotid gland abscess: A rare but serious condition where an abscess forms within the parotid gland, requiring prompt drainage and antibiotic treatment.