In this, the hair follicles are attacked by white blood cells. The follicles then become very small and hair production slows down dramatically, so there may be no visible hair growth for months and years.

After some time, hair may regrow as before, come back in patchy areas, or not regrow at all. The good news is that in every case the hair follicles remain alive and can be switched on again; the bad news is that it is not yet known how to do this.