Iritis, y RY tihs, is an inflammation of the iris, the colored part of the eyeball. The iris, a delicate membrane, determines the amount of light that passes through the pupil. Iritis can result from a variety of causes, including injury to the eyeball. It also may be associated with such diseases as chickenpox, mumps, rheumatoid arthritis, or tuberculosis. Iritis may occur by itself but may also accompany inflammation of other eye parts.