A person unduly concerned with health and often suffering from delusions of physical disease.
Example:
Hercule Poirot, the dapper hero of Agatha Christie's mysteries, is a notorious hypochondriac, always trying to protect himself from drafts.
One disease a hypochondriac really does suffer from is hypochondria, which is the mental depression that comes from worrying too much about health and is often accompanied by delusions of physical ailments. Somewhat surprisingly, hypochondria derives from hypo- and chondros, the Greek word for "cartilage." The cartilage in question is that of the sternum, or breastbone. From ancient times medical authorities had believed that certain internal organs or regions were the seat of various diseases, both physical and mental. The region beneath the centrally located breastbone was thought to be the seat of hypochondria.