Conjunctivitis: This is often the first consideration for a red and painful eye due to its high prevalence. It can be caused by viral, bacterial, or allergic factors, and its presentation includes redness, discharge, and sometimes pain.

Dry Eye Syndrome: This condition can cause eye pain and redness, especially if severe. It's due to insufficient lubrication of the eye.
Blepharitis: Inflammation of the eyelids can cause redness, itching, and pain in the eyes.
Corneal Abrasion: A scratch on the cornea can be very painful and cause redness, often resulting from trauma or foreign bodies in the eye.
Uveitis: Inflammation of the uvea, which includes the iris, ciliary body, and choroid. It can cause pain, redness, and sensitivity to light.

Acute Angle-Closure Glaucoma: A medical emergency that causes severe eye pain, redness, and vision loss. It's due to increased intraocular pressure.
Endophthalmitis: A severe infection within the eye that can cause pain, redness, and vision loss, often following eye surgery or trauma.
Orbital Cellulitis: An infection of the tissues surrounding the eye, which can cause pain, redness, swelling, and vision changes.
Retinal Detachment: Although it may not always cause pain, it can lead to sudden vision changes and requires immediate attention to preserve vision.

Scleritis: Inflammation of the sclera, which can cause severe pain, redness, and tenderness to the touch.
Keratitis: Inflammation of the cornea, which can be caused by infection or injury and leads to pain and redness.
Iritis: Inflammation of the iris, which can cause pain, redness, and sensitivity to light.
Sympathetic Ophthalmia: A rare, inflammatory condition that can occur in one eye after trauma or surgery in the other eye, leading to pain, redness, and vision loss.