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What it is: A condition activated by anything that triggers the body's defenses (such as a cut, burn or infection like strep throat), causing skin cells to run amok. Responding to a signal to heal, the body produces new skin cells more rapidly and in greater numbers than can be incorporated into the surface layer. The cells accumulate, creating flaky, itchy patches.

Symptoms: Small red bumps that progress to thick, scaly patches on your scalp, elbows, knees, hands and feet.

How to treat it: Though psoriasis is incurable, about 75 to 80 percent of the cases are mild, and treatment can diminish symptoms. Mild cases are treated topically with hydrocortisone, vitamin D- or vitamin A-derivative creams, and preparations that contain small amounts of coal tar, the oldest remedy for psoriasis. Sunlight also helps, but because of the risk of skin cancer, moderation is key. Severe cases of psoriasis are often treated with ultraviolet light in a doctor's office and/or oral retinoid medications.

Product recommendations: Neutrogena T/Gel Shampoo, $5, contains coal tar. The minerals in Ahava Dead Sea Bath Salts, $9, are a natural resource for combating irritation. Aquaphor Healing Ointment, $6, is fragrance- and preservative-free.