Cold dry weather is a common precipitant of psoriasis flares. Hot, damp, sunny weather helps relieve the problem in most patients. It should be noted, however, that some people have photosensitive psoriasis, which actually improves in winter and worsens in summer when exposed to sunlight. Stress, unexpressed anger, and emotional disorders, including depression and anxiety, are strongly associated with psoriasis flares. One study suggested that in patients with psoriasis the biochemical response to stress is different than it is in people who do not have the skin disease.
Infections caused by viruses or bacteria can trigger some cases of psoriasis.
Although experts have sought a bacterial or viral agent as a cause of pustular psoriasis, none have been identified thus far. In some cases, even mild abrasions can cause an eruption. Psoriasis can develop even in areas that were previously clear of the skin disease. Lithium may actually trigger the onset of the disease as well as cause severe flares in people who already have it. Because these drugs are also used to treat psoriasis, this rebound effect is of particular concern.