Psoriasis is a long-lasting autoimmune disease characterized by patches of abnormal skin.[6] These skin patches are typically reddryitchy, and scaly.[3] On people with darker skin the patches may be purple in colour.[9] Psoriasis varies in severity from small, localized patches to complete body coverage.[3] Injury to the skin can trigger psoriatic skin changes at that spot, which is known as the Koebner phenomenon.[10]

There are five main types of psoriasis: plaque, guttateinversepustular, and erythrodermic.[6] Plaque psoriasis, also known as psoriasis vulgaris, makes up about 90 percent of cases.[4] It typically presents as red patches with white scales on top.[4] Areas of the body most commonly affected are the back of the forearms, shins, navel area, and scalp.[4] Guttate psoriasis has drop-shaped lesions.[6] Pustular psoriasis presents as small non-infectious pus-filled blisters.[11] Inverse psoriasis forms red patches in skin folds.[6] Erythrodermic psoriasis occurs when the rash becomes very widespread, and can develop from any of the other types.[4]Fingernails and toenails are affected in most people with psoriasis at some point in time.[4] This may include pits in the nails or changes in nail color.[4]

Psoriasis is generally thought to be a genetic disease that is triggered by environmental factors.[3] In twin studiesidentical twins are three times more likely to be affected compared to non-identical twins. This suggests that genetic factors predispose to psoriasis.[4] Symptoms often worsen during winter and with certain medications, such as beta blockers or NSAIDs.[4] Infections and psychological stress can also play a role.[3][6]Psoriasis is not contagious.[4] The underlying mechanism involves the immune system reacting to skin cells.[4] Diagnosis is typically based on the signs and symptoms.[4]

There is no cure for psoriasis; however, various treatments can help control the symptoms.[4] These treatments include steroid creamsvitamin D3 cream, ultraviolet light and immune system suppressing medications, such as methotrexate.[6] About 75 percent of cases can be managed with creams alone.[4] The disease affects two to four percent of the population.[8] Men and women are affected with equal frequency.[6] The disease may begin at any age, but typically starts in adulthood.[5] Psoriasis is associated with an increased risk of psoriatic arthritislymphomascardiovascular diseaseCrohn’s disease and depression.[4] Psoriatic arthritis affects up to 30 percent of individuals with psoriasis.[11]


The United States of America (USA), commonly referred to as the United States (U.S.), America, and sometimes the States, is a federal republic[17][18] consisting of 50 statesand a federal district. The 48 contiguous states and Washington, D.C., are in central North America between Canada and Mexico. The state of Alaska is located in the northwestern part of North America and the state of Hawaii is an archipelago in the mid-Pacific. The country also has five populated and nine unpopulated territories in the Pacific and the Caribbean. At 3.80 million square miles (9.85 million km2)[4] and with around 318 million people, the United States is the world’s fourth-largest country by total area and third-largest by population. It is one of the world’s most ethnically diverse and multicultural nations, the product of large-scale immigration from many countries.[19]The geography and climate of the United States is also extremely diverse, and it is home to a wide variety of wildlife.[20]

The United States is a developed country and has the world’s largest national economy.[6] The economy is fueled by an abundance of natural resources and high worker productivity.[29] While the U.S. economy is considered post-industrial, it continues to be one of the world’s largest manufacturers.[30] The country accounts for 37% of global military spending,[31] being the world’s foremost economic and military power, a prominent political and cultural force, and a leader in scientific research and technological innovations.[32]