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Congenital (you are born w/o a sense of smell)
Viral (cold virus)
Head Injury (injury to olfactory bulb in the front of the head due to fall or car accident) direct injury to head or whiplash, which would cause your brain to bounce in your skull, severing the nerves which lead to the olfactory bulb.
Nasal Sprays (some zinc sprays have been linked to destroying the epithelial cells in the upper part of the nasal cavity, which communicate with the olfactory bulb in the brain).
Medications (some may be linked to loss of sense of smell) some people have reported using antihistamines as causing this condition.
Brain Tumour
Parkinson's and Alzheimer's diseases both report loss of sense of smell as an early sign of these diseases.