According to Dr Yeung Chi-keung, honorary clinical associate professor at the University of Hong Kong, psoriasis affects 0.3 per cent of the Hong Kong population, compared to a worldwide rate of 1 to 2 per cent.

The incurable and poorly understood condition is thought to be related to a malfunction in the immune system, in which the body mistakes skin cells as a threat and attacks them. The immune system also causes the skin cells to divide much faster than the normal rate, and dead cells start to pile up on the skin surface as the body is unable to clear the excess skin cells quickly enough.

While there is a strong genetic component - as in Lai's case - psoriasis is not contagious. Doctors also believe certain triggers must be present in combination with the genetic disposition before psoriasis flares.

Psoriasis can take a number of forms, from mild to severe, appearing as small drop-like spots, to plaque-like patches with silvery scales and pus-filled blisters.

Lai had the most common type - the plaque-like patches called psoriasis vulgaris. But his condition became severe, covering most of his body. Finding relief was a frustrating and winding journey for Lai. Because psoriasis is incurable, the goal of treatment is often to control symptoms and prevent complications.

Topically, steroid creams are often used as a first line treatment to reduce the inflammation and redness caused by psoriasis. However, given the extensive coverage of psoriasis on Lai's body, it was not a viable solution. Other common topical treatments include skin lotions, creams, ointments and shampoos that contain coal tar, salicylic acid and vitamin D- or vitamin A-derived products.

Lai then tried oral medications that aim to suppress the body's faulty immune response. One of them, methotrexate, affected his liver, leading to raised liver enzyme levels, so he was forced to stop taking the medicine.

Doctors told him to use light therapy where ultraviolet B light (UVB) is used to slow down the inflammation of the skin.

Unfortunately, for the phototherapy to have been effective, Lai would have needed to visit a doctor's office for the therapy twice a week during office hours. He could not do this without creating problems at work and had to forego this option.

Retinoids eventually helped to control his condition by up to 50 per cent by slowing the growth and shedding of skin cells. But Lai continued to see it flare and recede, often according to the seasons. Winter is a particularly aggravating time for psoriasis sufferers.

Lai then started to participate in clinical trials that tested therapies and new drugs on psoriasis patients. The most effective treatment to date is a type of medicine called biologics - bioengineered protein-based drugs that target specific parts of the immune system.

During the biologics drug trials, Lai saw a 75 per cent improvement in his condition, much to his delight. Unfortunately, the drug can be prohibitively expensive at nearly HK$10,000 a month, and is not covered by government aid. Lai returned to using retinoids after the biologics trials ended.

The effects of psoriasis are not limited to physical and financial aspects. Yeung says that psoriasis has a strong psychological impact on sufferers because it is a disfiguring disease. A patient's self-image and self-confidence can take a heavy beating.

In the early years, after the initial onset of psoriasis, Lai sank into depression and hid himself from the world. He was afraid to meet his friends for fear of their reactions to his condition.

Although Lai continued to work, psoriasis cast a heavy cloud over his career. The maddening itch that he suffered robbed him of his sleep at night and his concentration during the day.

Sometimes, Lai was driven to scratch vigorously, so much so that his already vulnerable skin would rip and bleed. The blood would soak through and stain his clothes, drawing looks of suspicion and alarm from bystanders and colleagues.

Well-meaning colleagues would enthusiastically ply him with suggestions to seek alternative treatments such as traditional Chinese medicine or acupuncture. But all the unwanted attention only added to the pressure he felt. Other colleagues shunned him.

Reactions from people on the streets were less kind. Fellow travellers on the MTR would give him a wide berth for fear that they might "catch" the disease. Swimmers at public swimming pools would urge the lifeguards to stop Lai from entering the water because they did not want him to "contaminate" it.

Yeung emphasises that psoriasis is not a contagious disease but an autoimmune one, and hopes that the general public will become more accepting and understanding of psoriasis patients.

Lai now makes it his life's mission to help other patients and to educate the public about psoriasis. He is a member of support groups and acts as an advocate for psoriasis sufferers. He encourages psoriasis patients to receive proper treatment and group support.