Complex sentence: Those suffering from conjunctivitis may report mild irritation or scratchiness, but never extreme pain, which is an indicator of more serious disease such as keratitis, corneal ulceration, iridocyclitis, or acute glaucoma.
Split into simpler sentences: Those suffering from conjunctivitis may report mild irritation or scratchiness, but never extreme pain. Severe pain is an indicator of keratitis, corneal ulceration, iridocyclitis, or acute glaucoma.