Persistent genital arousal disorder (PGAD) is a newly described disorder that is not yet fully understood or even defined. It was originally called persistent sexual arousal syndrome (PSAS) and was first written about in the clinical literature in 2001, although it is likely something that has been experienced by women (and possibly men, although none have been identified yet) for many years.
Persistent genital arousal disorder describes the experience of persistent feelings of genital arousal that is not caused by sexual stimulation and is not experienced as sexual pleasure. These feelings can last for hours or days and can cause extreme distress and discomfort to the person experiencing PGAD.
While some laypeople and medical professionals have misunderstood PGAD and considered suffers "lucky" (assuming that everyone would want to feel sexually aroused constantly, and that all genital arousal is sexually pleasurable) in reality PGAD can come with broad negative sexual consequences, and people who experience it can suffer from depression and even thoughts of suicide as a result of the experience.
Research is only in the earliest stages of understanding PGAD, and at this point no single cause or treatment method has been defined, although there are several ways of reducing the negative impact of PGAD.
Leiblum, S.R. "Persistent Genital Arousal Disorder: What It Is and What It Isn't." Contemporary Sexuality. Vol. 40, No. 10 (October 2006), 8-13.