An estimated 1-3% of the world’s population has psoriasis at present. Onset can at any age, though the early onset implies a less stable, more severe clinical course. Age of onset peaks during 20s and again in late 50s.
There are known inherited genetic factors and several documented environmental factors that affect the clinical course of psoriasis. Men and women are equally affected. Siblings and offspring are at increased risk of developing psoriasis.
Factors which may exacerbate psoriasis include human immunodeficiency virus (HIV) infection, physical trauma, infections including Streptococcus and Candida, certain drugs such as lithium, beta-blockers, antimalarials, and systemic corticosteroids.
The severity of psoriasis varies widely from one patient to another and over time. Severity is graded by the degree of skin involvement and the presence of joint involvement. A patient is considered to have severe psoriasis when more than 10% of the body surface is involved or when psoriatic arthritis is present.
One-third of patients have nail involvement. Psoriatic arthritis (rheumatoid factor negative) is found in 5% of the psoriatic population. Onset may precede, accompany, or follow skin manifestations.