The causes of benign tumors in the parotid glands, the largest salivary glands, are unknown, although risk factors for cancerous tumors are known, according to the American Cancer Society website. The most common types of parotid tumors, pleomorphic adenoma and Warthin's tumor, are usually benign, slow growing and not life threatening.
Benign parotid gland tumors are usually surgically removed. Some tumor-sites are then treated with radiation to prevent recurrence. If a benign tumor is left untreated or incompletely removed, it may, in rare cases, recur as a malignant tumor, states cancer.org. The risk factors for malignant tumors of the parotid glands are male gender, older age, radiation exposure, environmental factors and family history.
In addition to tumors, other conditions may cause swelling of the parotid glands. Crystallized saliva deposits can cause stones to develop in the saliva duct and block the tube. Saliva then backs up into the gland, which leads to pain, swelling and infection, states WebMD.
Another cause of swelling is bacterial infection, known as sialadenitis. Bacterial infection in the salivary gland can block the duct that leads to the mouth, also causing saliva to back up into the gland, reports WebMD. Finally, viral infections can cause the glands to swell. Mumps, flu and other viruses can cause the parotid glands to swell.