Research indicates that the disease may result from a disorder in the immune system. The immune system makes white blood cells that protect the body from infection. In psoriasis, the T cells (a type of white blood cell) abnormally trigger inflammation in the skin. These T cells also cause skin cells to grow faster than normal and to pile up in raised patches on the outer surface of the skin.

Those with a family history of psoriasis have an increased chance of having the disease. Some people carry genes that make them more likely to develop psoriasis. When both parents have psoriasis, the child may have a 50% chance of developing psoriasis. About one third of those with psoriasis have at least one family member with the disease.

Certain factors may trigger psoriasis.

Injury to the skin: Injury to the skin has been associated with plaque psoriasis. For example, a skin infection, skin inflammation, or even excessive scratching can trigger psoriasis.
Sunlight: Most people generally consider sunlight to be beneficial for their psoriasis. However, a small minority find that strong sunlight aggravates their symptoms. A bad sunburn may worsen psoriasis.
Streptococcal infections: Some evidence suggests that streptococcal infections may cause a type of plaque psoriasis. These bacterial infections have been shown to cause guttate psoriasis, a type of psoriasis that looks like small red drops on the skin.
HIV: Psoriasis typically worsens after an individual has been infected with HIV. However, psoriasis often becomes less active in advanced HIV infection.
Drugs: A number of medications have been shown to aggravate psoriasis. Some examples are as follows:
- Lithium - Drug that may be used to treat depression
- Beta-blockers - Drugs that may be used to treat high blood pressure
- Antimalarials - Drugs used to treat malaria
- NSAIDs - Drugs, such as ibuprofen (Motrin and Advil) or naproxen (Aleve), used to reduce inflammation
Emotional stress: Many people see an increase in their psoriasis when emotional stress is increased.
Smoking: Cigarette smokers have an increased risk of chronic plaque psoriasis.
Alcohol: Alcohol is considered a risk factor for psoriasis, particularly in young to middle-aged males.
Hormone changes: The severity of psoriasis may fluctuate with hormonal changes. Disease frequency peaks during puberty and menopause. A pregnant woman's symptoms are more likely to improve than worsen, if any changes occur at all. In contrast, symptoms are more likely to flare in the postpartum period, if any changes occur at all.