Psoriasis is a common skin condition that causes skin redness and irritation. Most persons with psoriasis have thick, red skin with flaky, silver-white patches called scales.

See also: Guttate psoriasis

Psoriasis is a very common condition. The disorder may affect people of any age, but it most commonly begins between ages 15 and 35. It can appear suddenly or slowly. In many cases, psoriasis goes away and then flares up again repeatedly over time. The condition cannot be spread to others.

Psoriasis seems to be an inherited disorder. That means it is passed down through families. Doctors think it probably occurs when the body's immune system mistakes healthy cells for dangerous substances. See also: Inflammatory response

Skin cells grow deep in the skin and normally rise to the surface about once a month. In persons with psoriasis, this process is too fast and dead skin cells build up on the skin's surface.

The following may trigger an attack of psoriasis or make the condition more difficult to treat:

• Bacteria or viral infections, including strep throat and upper respiratory infections
• Dry air or dry skin
• Injury to the skin, including cuts, burns, and insect bites
• Some medicines, including antimalaria drugs, beta-blockers, and lithium
• Stress
• Too little sunlight
• Too much sunlight (sunburn)
• Too much alcohol

In general, psoriasis may be severe in persons who have a weakened immune system. This may include persons who have:

• AIDS
• Autoimmune disorders (such as rheumatoid arthritis)
• Cancer chemotherapy

Up to 30% of people with psoriasis may also have arthritis, a condition known as psoriatic arthritis. Psoriasis may also affect the nails. About 10% of people with psoriasis have visible changes only in the nails.