Seborrheic Dermatitis is not necessarily a symptom of HIV, but it is thought to be related to the people carrying HIV, because only 2 to 3 percent normal people are said to have this disease, where as around 85% HIV patients develop Seborrheic Dermatitis. It's because of their vulnerability to the diseases. Anyhow, a person having Seborrheic Dermatitis should go for the HIV test, just to clarify the reason behind this disease.
