Seborrheic dermatitis

Seborrheic dermatitis is a common skin disease. It's inflammatory, which means the affected skin is irritated and reacting. It shows up right where the skin has the most sebaceous glands, the tiny glands that make sebum, your skin's natural oil: the scalp, the face, and the folds of the body. In those spots it forms plaques, salmon-colored patches of skin covered by a crust of scales that looks greasy.

What's going on underneath has to do with what lives on your skin. Skin is normally home to microorganisms, living things too small to see without a microscope, that make up the skin's normal flora. Malassezia is one of them. Seborrheic dermatitis is linked to the way these come together: those microorganisms; lipids, the fats that coat the surface of the skin; and each person's susceptibility, meaning how prone their skin is to react. Malassezia breaks down sebum and throws off the balance of those fats. Think of the skin's oily layer as a well-mixed salad dressing: if something starts breaking down one of its ingredients, the mix doesn't stay the same. That's what happens to the lipids on the skin's surface.

There are two forms. The infant form, the one babies get, is mild and usually goes away on its own within the first year of life. The adult form is chronic. It comes and goes, with relapses, stretches when it gets worse, and remissions, stretches when it settles down.