Conjunctivitis: This is the most common cause of a red, swollen eye, often due to viral or bacterial infection, or allergic reactions. It presents with redness, swelling, discharge, and sometimes itching or burning sensation.

Blepharitis: Inflammation of the eyelids can cause redness, swelling, and sometimes a gritty sensation in the eyes. It's often associated with skin conditions like acne or eczema.
Dry Eye Syndrome: Insufficient tear production or poor tear quality can lead to eye irritation, redness, and swelling.
Corneal Abrasion: A scratch on the cornea can cause pain, redness, and swelling, often accompanied by sensitivity to light and blurred vision.
Uveitis: Inflammation of the uvea, the middle layer of the eye, can cause eye pain, redness, and swelling, along with sensitivity to light.

Acute Angle-Closure Glaucoma: A medical emergency characterized by sudden, severe eye pain, redness, swelling, and vision loss. It requires immediate treatment to prevent permanent vision loss.
Endophthalmitis: A severe infection within the eye that can cause pain, redness, swelling, and vision loss. It's a rare but potentially devastating complication of eye surgery or trauma.
Orbital Cellulitis: An infection of the tissues surrounding the eye can cause swelling, redness, and pain, and can lead to serious complications if not promptly treated.

Keratitis: Inflammation of the cornea, which can be caused by infection, injury, or underlying conditions like rheumatoid arthritis.
Scleritis: Inflammation of the sclera, the white part of the eye, which can cause severe pain, redness, and swelling, often associated with systemic autoimmune disorders.
Ocular Migraine: A condition that can cause temporary vision loss, pain, and redness, often accompanied by migraine headaches.