psoriasis is may b cause by some bacteria and if any one in your family has this type of infection please do not use his cloths and soap it can spread to you easily if your skin cells also weaker like the patient.
Some scientist said that abnormal immune system is the main reason for this disease when our immune system fail to produce healthy cells so this condition lead to infection in skin and skin loss its original structure.
Science has not found any cure yet for this disease but i believe if a person has dead cells new stem cell transplant may help to cure this problem.We can hope in future there will be treatment available for these life threaten diseases.