For many psoriasis sufferers, winter brings not only shorter days and colder temperatures but also worsening symptoms.

"About 70 percent of psoriasis patients will experience exacerbated symptoms at this time of year," says Bai Yanping, a skin problem specialist at Beijing's China-Japan Friendship Hospital.

Dry air, colder temperatures and reduced exposure to sunlight can all worsen symptoms, Bai explains.

Psoriasis is caused by a mix of environmental and genetics factors and can appear suddenly or gradually. In many cases, psoriasis goes away and then reoccurs.

Most people with the irritation have thick, red skin with flaky, silver-white patches on the elbows, knees and trunk, but it can appear anywhere on the body, including the scalp.

Psoriasis sufferers, particularly young people, can also experience anxiety when they suffer from attacks, which makes the situation worse.

"Stress can worsen the situation, and create a vicious circle," Bai says.

"The itchiness almost kills me. I know I shouldn't scratch the sores, but I can't help it," says Zhao Jinsheng, a 50-something Beijinger who has psoriasis all over his body.

"Even if you offered me 1 million yuan ($157,131) you could not stop me scratching."

Although the ultimate cause of the disease is still unknown, doctors believe psoriasis is an autoimmune disease and probably occurs when the body's immune system mistakes healthy cells for dangerous ones.

"When the disease attacks, the produce of cytokine will be abnormally stimulated, causing inflammation and the rapid production of certain skin cells, as well as the dilation of blood vessels," Bai says.

"As a result, the skin will become red and thick."

Ultraviolet light can slow the growth of the skin cells, and ultraviolet therapy is used in the treatment of psoriasis, Bai says.

Sunlight also causes the body to produce vitamin D naturally, which is good for psoriasis sufferers, and is better than a vitamin D supplement.

"The best way to combat psoriasis is make your body as healthy as possible," Bai says. "Having a balanced diet, doing exercise and staying in good mood are our most effective defense against the disease."

For average psoriasis sufferers, a short and mild-temperature shower is beneficial, while for those with more serious symptoms, a bath with prescribed traditional Chinese medicine is advised, Bai says.