Most cases of conjunctivitis affect both eyes. Allergic conjunctivitis is most often bilateral, meaning that it occurs in both eyes. Bacterial and viral conjunctivitis often begin in one eye and quickly spread to the other eye. By the time someone goes to the doctor for diagnosis and treatment, bacterial and viral conjunctivitis is probably affecting both eyes.

There are cases of pink eye that affect only one eye. When someone has severe conjunctivitis in one eye, it may be called acute unilateral conjunctivitis. Acute unilateral conjunctivitis can have several causes.
Unilateral conjunctivitis can be attributed to a medical cause. In a few cases, acute unilateral conjunctivitis has been found to be the result of a vaccine. At least one case has been attributed to a reaction of the rubella vaccine.

Another possible cause of unilateral conjunctivitis is if the conjunctivitis is due to a complication from eye surgery. A patient who has had eye surgery may develop conjunctivitis due to an infection at the site of the surgery. Others may develop conjunctivitis from sensitivity to any sutures used on the eye. Such an irritation as can cause giant papillary conjunctivitis.

Unilateral conjunctivitis could indicate a physiological problem with the conjunctiva or cornea. Damage to the conjunctiva from an injury or trauma could cause conjunctivitis in the affected eye. Keratitis is damage to the cornea. If someone sustains damage to the cornea and develops keratitis, the patient often has symptoms of conjunctivitis as well. The condition may be diagnoses as keratitis conjunctivitis.

In rare cases, unilateral conjunctivitis can be caused by cancer. Squamous cell carcinoma is the most common cancer that affects the conjunctiva, the outer membrane of the eye. Squamous cell carcinoma is most common among Caucasian males. It most commonly affects people over the age of sixty and people with acquired immunodeficiency syndrome (AIDS). People who live near the equator or are exposed to ultraviolet light are at increased risk. The affected eye can have patches of redness or an overall reddish color as is common with conjunctivitis.

When someone experiencing unilateral conjunctivitis is examined by a doctor, the doctor is likely to ask questions to obtain a personal history. The doctor may pay particular attention to any history of eye injury, eye surgery, history of conjunctivitis, the patient’s use of over-the-counter eye medication, contact lens use, and recent medication and medical procedures. A physician may refer the patient to an ophthalmologist, an eye specialist. If the cause is unknown, the physician may ask an ophthalmologist to rule out obstruction of the tear duct as the cause.