Genetic predisposition to psoriasis is not necessarily a genetic fate. Genetic predisposition establishes a set of conditions that may be necessary to cause psoriasis, but are not in and of themselves sufficient to cause psoriasis. Psoriasis may result, in some persons, only when a genetic predisposition is "triggered" by an environmental event.

Environmental triggers may be:

• The provocation for the initial appearance of psoriasis;
• Factors that cause new outbreaks of psoriatic lesions while the disease is in an active phase; or,
• Factors that worsen psoriasis while the disease is in an active phase.

Environmental triggers that have been identified range from climate to physical injury (trauma) to infection to psychological stress:

Climate and Weather
Epidemiologic studies indicate that cold weather may be a predisposing effect or trigger for psoriasis, in contradistinction to hot and sunny climate that appears to be beneficial.

Physical Trauma and Other Skin Conditions
In the 19^th Century, a Dr. Koebner described a patient with psoriasis who developed new lesions at spots where he was bitten by his horse. The relationship between physical trauma and psoriatic lesions was subsequently documented in many more patients and was given the name "Koebner’s phenomenon". Analysis of patient records has indicated that up to 50 percent of persons with psoriasis have had a "Koebner’s phenomenon" experience—that is, have had a psoriatic lesion develop at the site of an injury or skin condition. In perhaps about 10 percent of psoriasis patients every injury or skin condition causes a new psoriatic lesion to develop at the site of injury. The likelihood of Koebner’s phenomenon is probably increased during an active phase of the disease.

A broad range of injuries and skin conditions has been linked with Koebner’s phenomenon:

Infections
Both dermatologic and systemic infections have been known to trigger the onset of psoriasis or a worsening of psoriasis. The incidence of infectious triggering has varied from study to study, but the triggering effect of infections is well documented. Systemic infections that have been associated with triggering include viral upper respiratory disease, streptococcal pharyngitis ("strep throat"), and human immunodeficiency virus (HIV). Staphylococcal skin infections (boils) have been a trigger.

Psychological Stress
It is difficult to separate job and family-related stress from the psychological stress of living with psoriasis. One cause of stress probably reinforces the others. Nevertheless, the perception of patients that psychological stress can worsen psoriasis has been supported in clinical studies. Patients may be tempted to reduce stress by abusing drugs or alcohol—activities that can actually increase stress. A stress-depression, stress-aggression, or stress-obsession pattern may be accompanied by increased pruritus (itching). A psoriasis patient who feels overwhelmingly stressed should ask the dermatologist to arrange for psychological counseling, which may include members of the family.

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