The taste cells are clustered in the taste buds of the mouth and throat. Many of the small bumps that you can see on your tongue contain taste buds.

If you hold your nose while eating chocolate, for example, you will have trouble knowing it is a chocolate flavor, even though you can sense the food's sweetness or bitterness. That's because the familiar flavor of chocolate is sensed largely by odor. So is the well-known flavor of coffee.

Smell disorders are more common. A loss of smell is the most common problem, but other problems include misreading or distorting an odor. Smell and taste disorders rarely occur together.

Many people do develop smell and taste problems after an injury or illness, but some people are born with them. Upper respiratory infections are blamed for some chemosensory losses. A head injury can also cause smell or taste problems. Changes also occur as you age and as a side effect to certain medicines.

Other causes of taste and smell disorders include polyps in the nasal cavities, sinus infections, and dental problems. Loss of smell and taste also can be caused by exposure to certain chemicals such as insecticides and by some medicines. Radiation therapy for cancers of the head and neck also can cause chemosensory problems. Smoking also causes smell disorders.

The person taking the test scratches pieces of paper treated to release different odors, sniffs them, and tries to identify each odor from a list of choices.

Problems with smell or taste can leave you without an early warning system that most people take for granted. Smell and taste alert you to fires, poisonous fumes, leaking gas, and spoiled food and beverages. Smell and taste losses can also lead to depression.

Chemosensory problems may point to illnesses such as obesity, diabetes, high blood pressure, certain viral infections, and malnutrition. They may also point to some degenerative diseases of the nervous system such as Parkinson disease, Alzheimer disease, and Korsakoff psychosis.