A viral upper respiratory infection is one of the most commonly identified causes of olfactory loss, accounting for 20% to 30% of patients in most series. Given the ubiquitous nature of upper respiratory infections, it is not clear what predisposes some patients to develop this complication. Studies have demonstrated degenerative changes within the olfactory epithelium, the severity of which seems to correlate with the severity of olfactory loss. Although no available therapy has proved effective, long-term follow-up data have found that approximately two thirds of these patients eventually experience a significant improvement in their olfactory function.