Why is only one eye red?

There are many possible causes for one eye to be red, including:

* Conjunctivitis, also known as pink eye, is a common infection of the conjunctiva, the clear membrane that covers the white part of the eye. It can be caused by viruses, bacteria, or allergies.

* Corneal abrasion, also known as a scratched cornea, is a break in the clear outer layer of the eye. It can be caused by foreign objects, such as dirt or dust, or by contact lenses.

* Iritis, also known as inflammation of the iris, is a condition in which the colored part of the eye becomes inflamed. It can be caused by autoimmune disorders, infections, or injuries.

* Subconjunctival hemorrhage, also known as a broken blood vessel in the eye, is a condition in which a small blood vessel bursts under the conjunctiva. It can be caused by trauma, coughing, sneezing, or straining.

* Stye, also known as a hordeolum, is a small, red, painful bump on the eyelid. It is caused by a bacterial infection of the glands that produce oil for the tears.

If you have one eye that is red, it is important to see a doctor to determine the cause and get the appropriate treatment.