One symptom can be a lifetime without a sense of smell, called congenital anosmia.

BBC: Gene therapy restores sense of smell in mice

Anosmia, an inability to smell, affects an estimated three million people in the UK and can be present from birth.

BBC: James Paget Hospital smell clinic 'victim of own success'

Matrixx had failed to disclose a dozen or so cases of anosmia, or loss of smell, associated with its popular Zicam Cold Remedy.

FORBES: 'Pro-Business' Supreme Court Defies Critics Again

From 3 million to 4 million Americans have been diagnosed with conditions such as anosmia, the complete inability to smell, and hyposmia, a reduced capacity, Dr. Hirsch estimates.

WSJ: How to Sharpen Your Sense of Smell