This is uncommon. The Iris is the colored part of our eye (blue / brown / hazel / etc.). The Uveal Tract is the same color, but it’s under the sclera all around the eyeball, so we can’t see it (see Diagram — Anatomy of the Eye). Iritis is sometimes called “Anterior Uveitis” (the front part of the Uvea). Iritis may just happen on its own, may be due to certain infections (syphilis, TB, Lyme, Mono, tropical germs, etc.), or may be a part of general inflammatory diseases like rheumatoid arthritis, Crohn’s, etc. etc.
The eye usually just looks red (conjunctiva react to the inflammation); sometimes the area right around the iris may seem the reddest. The pupil may be constricted, or may be dilated wide in spasm. But the main thing that alerts us is that there’s significant eye pain.
If both pupils look the same, a maneuver called “cross-photophobia” can clue us in to Iritis — shining a bright light in the good (healthy) eye makes the bad (diseased) eye hurt! We know that when light hits one eye, both pupils constrict. The only ocular structure which does anything when light shines in the other eye is the Iris, which manipulates the pupil. So if bright light in the good eye causes pain on the other, we know that something is happening to the Iris. (Most clinicians don’t know this).
An ophthalmologist or optometrist with a slit lamp can make the diagnosis for sure, but only the former can begin to figure out the cause. Treatment obviously depends on the underlying disease. Some cases get better on their own, some wind up with severe damage.