This disease is an inflammation of the uvea (the middle layer of the eye). Uveitis is primarily defined by which side of the uvea is affected. Anterior uveitis (aka iritis) refers to the inflammation being on the outermost side, while posterior uveitis (aka choroiditis) describes the affected inside layer.
When both sides of the uvea are inflamed it is called pan-uveitis. No matter which form the illness takes, inflammatory cells can enter the gelatinous-like center of the eye and spread. In most case scenarios the infection occurs suddenly and spreads quickly. Uveitis can affect one or both eyes and depending on cause can be infectious or non-infectious.