Jealousy is not just an extreme reaction to real or imagined infidelity, but any maladaptive jealous emotion. At one extreme there is morbid jealousy, with which the symptoms may be close to psychosis (as in the movie "Fatal Attraction"). Sufferers interrogate the other, may spy on them, hire a private detective, or try to prevent their activities alone, even to the extend of murder. At the other extreme, there are people who are never jealous.
There is no typical jealous personality. Men and women get equally jealous, but they do differ in the matter of motivation. Men react more to sexual infidelity and their jealousy can develop more easily into into a psychotic variety. Women suffer more with suspicions of emotional involvement and can sustain longer periods of jealousy. Women's jealousy is also easily linked to insecurity, driven perhaps by the biological imperative of needing a faithful mate in order to raise children. Between 10 and 15% of people seeking sexual and marital therapy report jealousy as one part of their problem.
Jealousy is an emotional condition of delusionary romantic love. It arises from the need to exclusively possess another human being and to derive the satisfaction of all one's needs from that person. In polygamous societies, jealousy is virtually non-existent.
Mild jealousy is part of love and bonding with a partner, and therefore healthy.