Nail Fungus: Also known as Onychomycosis which is a fungal infection of the nails.

Narcissism: Also known as Narcissistic Personality Disorder.  These individuals have a pattern of grandiosity and need for admiration.  They lack empathy.  These individuals are generally full of themselves and exaggerate their self importance.  See related article on Narcissistic Personality Disorder

Narcolepsy: Frequent, brief and uncontrollable periods of sleep.

Nicotine: The addictive drug in tobacco. See related article on Teen Smoking