A nega in the beard or hair, called a nethek נתק, may be what is commonly known as AA or Alopecia areata.
The nethek is a bald patch (not caused by hair picking or shaving) the size of a griss, that may be indicative of immediate tumah, provided that it has at least two thin blond hair strands in that bald patch area. Generally, nethek isn’t permanent -unlike natural baldness which is often permanent- the nethek area still contains the ability to produce hair and may do so at a later time (upon its healing).
It is a negative mitzvah -מצוות לא תעשה- to shave or pick out any gold hairs that grow within the nethek (Sifra tazria min. Ch, 133).
If this said bald patch appears anywhere on the body but the hair and/or beard, it is not an indication of tumah. (Sifra tazria min. Ch. 124)
The rambam’s (commentary to mishna negaim 10 5 and ר״ש to negaim 10:2) understanding is that the bald patch area in the beard or on the head needn’t have the ‘four shades of white’ coloring to it, meaning just the bald patch and two blond hairs are grounds for immediate pronunciation by the kohen as tammei.(Sifra tazria min. Ch. 124 פירוש התורה והמצוה).
The hair strand is a tumah indicator only when golden-colored, neither white or red are indicative of tumah in terms of the head and beard (sifra) and two black, red, (or green) strands appearing on the נתק in tandem with the golden hair exempts it from tumah (rambam to mishna nega’im 10:2, Sifra tazria min. Ch. 137), with that said, one black, red or green hair isn’t enough to avoid a tumah call (Sifra tazria min. Ch. 126).
If a pisyon occurs after the kohen’s initiating a seven-day quarantine, the affected person is labeled by the kohen as tammei. Even if the pisyon occurs after the affected person is deemed tahor by the kohen- say three months later- the person is pronounced tammei by the kohen ( Sifra tazria min. Ch. 134 Mishna nega’im 10 1)
All the beard
Although only the thicket if the beard is the part one would be liable for regarding the prohibition of השחתת הזקן regarding the beard nega, the entire beard area is candidate (Sifra tazria min. Ch. 120 see pirush hatorah vehamitzvah).
AA has similar characteristics to the nethek as described in the torah; Although AA usually produces bald patches that are completely smooth, on close inspection of the bald patch, it very often produces visibly finer hair that are often lighter than the affected persons normal hair color.