The rash in chicken pox first appears as pinkish raised bumps a few millimetres across, usually on the chest or abdomen.
Within hours the bumps develop into very itchy blisters, containing a clear fluid. These blisters are known as vesicles.
The vesicles break down quite rapidly and form crusts and scabs, but a new crop of vesicles appears just as the previous crop starts to crust.
Typically 250 to 500 vesicles will form during the chicken pox illness. The chicken pox rash is usually mostly clustered on the chest and abdomen, with fewer vesicles on the face and limbs.
Vesicles on the scalp may be accompanied by swollen lymph glands at the back of the neck. Along with the skin blisters, blisters may also occur in the mouth and throat, on the eyelids, and in the genital and anal areas. In these areas, the blisters tend to break down, forming painful ulcers.
Blisters in the mouth make eating or drinking difficult. The blisters usually stop forming by the 5th day of the rash, and most of these become crusted by the 6th day of the illness.
Reviewed by paediatrician Prof Eugene Weinberg. MBChB; FCP (SA); PAED (SA). March 2018.