Viral Parotitis (Mumps): This is the most common cause of bilateral parotid swelling, especially in children and young adults. The sudden onset and bilateral involvement are characteristic features.

Bacterial Parotitis: Can cause sudden onset of parotid swelling, often accompanied by fever, pain, and purulent discharge from the parotid duct. It's more common in elderly or immunocompromised patients.
Sialadenitis: Inflammation of the salivary glands can be caused by various factors, including stones, infection, or autoimmune diseases, leading to sudden swelling.
Allergic Reaction: Certain allergic reactions can cause parotid gland swelling, though this is less common.

Sjögren's Syndrome: An autoimmune disorder that can cause parotid swelling among other symptoms like dry eyes and mouth. It's crucial to diagnose due to its implications on the patient's quality of life and potential for other systemic manifestations.
Lymphoma: Though rare, lymphoma can present with parotid swelling. Given its severity, it's a diagnosis that must not be missed.
Parotid Abscess: A serious condition that requires prompt treatment to avoid complications like sepsis or facial nerve damage.

Heerfordt's Syndrome (Uveoparotid Fever): A form of sarcoidosis that affects the parotid gland, uvea, and sometimes the facial nerve, presenting with parotid swelling, uveitis, fever, and sometimes facial palsy.
IgG4-Related Disease: A systemic fibroinflammatory disease that can affect the parotid glands, among other organs, leading to swelling.
Warthin's Tumor: A benign tumor of the parotid gland, which can cause swelling, though it typically presents unilaterally and with a more gradual onset.