The nerve endings for your sense of smell start in the upper spaces of the nose. When air passes through your nose, some if it usually flows up around these nerve endings and we smell the scents in the air. If the nerve endings are damaged, then we can lose a part or all of our sense of smell.

One common cause for a total loss of smell is infection. Most often these are viral infections, such as the common cold. Bacterial infections such as scarlet fever can cause the loss as well. If your sense of smell doesn't come back within 6 months of the loss, it is unlikely that it will return.

Anything that blocks the flow of air to the upper part of the nose can also cause a loss of smell. Common causes are nasal allergies and polyps. Often, treating these problems can bring the smell back.

Trauma can cause loss of smell, too. For instance, head injuries or nasal surgery can sometimes cause damage to the nerve endings as they pass through the fine bony openings between the brain and the nose.

Tumors of the nerve itself can also lead to the loss. But this is rare. The loss of smell caused by a tumor usually occurs gradually over many weeks or longer.