Psoriasis occurs when skin cells are replaced more quickly than usual. It's not known exactly why this happens, but research suggests it's caused by a problem with the immune system. Your body produces new skin cells in the deepest layer of skin.

What is the main cause of psoriasis?

Psoriasis is caused, at least in part, by the immune system mistakenly attacking healthy skin cells. If you're sick or battling an infection, your immune system will go into overdrive to fight the infection. This might start another psoriasis flare-up. Strep throat is a common trigger.

Where does psoriasis usually start?

Usually starting as small red bumps on the skin, plaque psoriasis (pictured) develops into red patches with a silvery, scaly coating — these raised patches are called plaques. Plaques usually show up on elbows, knees, and the lower back, and they can last for months or even years without treatment.