The most common illnesses that cause a loss of smell, or anosmia, include a sinus infection, the common cold, influenza, hay fever and nonallergic rhinitis, according to the Mayo Clinic. Nonallergic rhinitis is a condition that results in sneezing and congestion.
In some cases, a loss of smell can result from nasal growths, bone deformities and tumors, states the Mayo Clinic. Sometimes anosmia is a symptom of a more serious medical problem such as Alzheimer's, diabetes, Huntington's disease, Parkinson's disease and multiple sclerosis. It can also occur with age and malnutrition or after radiation therapy. If the loss of smell occurs due to infection, colds or allergies, the situation resolves itself within a few days or a week. If it doesn't go away in a timely fashion, the individual should consult a doctor to determine the cause of the problem and rule out more serious illnesses.
Treatments for anosmia vary depending on the reason for the loss of smell, says WebMD. For instance, if the cause is nasal polyps or growths, the polyps need to be surgically removed. In some cases, loss of smell is a side effect of certain medications. These medications need to be discontinued, but only under the direction of a doctor. Sometimes smoking can dull the nose. The solution is to quit smoking.