Causes

The most common causes of conjunctivitis (pink eye) are

• viruses
• bacteria
• allergens

Other causes include

• chemicals
• contact lens wear
• foreign bodies in the eye (like a loose eyelash)
• indoor and outdoor air pollution caused, for example, by smoke, dust, fumes, or chemical vapors
• fungi
• ameba and parasites

It can be difficult to determine the exact cause of conjunctivitis because some symptoms may be the same no matter the cause.

Viral Conjunctivitis

• Infection of the eye caused by a virus
• Can be caused by a number of different viruses, such as adenoviruses
• Very contagious
• Sometimes can result in large outbreaks depending on the virus

Bacterial Conjunctivitis

• Infection of the eye caused by certain bacteria
• Can be caused by Staphylococcus aureus, Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis, or, less commonly, Chlamydia trachomatis and Neisseria gonorrhoeae
• Can be spread easily, especially with certain bacteria and in certain settings
• Children with conjunctivitis without fever or behavioral changes can usually continue going to school
• More common in kids than adults
• Observed more frequently December through April

Allergic Conjunctivitis

• The result of the body’s reaction to allergens, such as pollen from trees, plants, grasses, and weeds; dust mites; molds; dander from pets; medicines; or cosmetics
• Not contagious
• Occurs more frequently among people with other allergic conditions, such as hay fever, asthma, and eczema
• Can occur seasonally, when allergens such as pollen counts, are high
• Can also occur year-round due to indoor allergens, such as dust mites and animal dander

Conjunctivitis Caused by Irritants

• Caused by irritation from a foreign body in the eye or contact with smoke, dust, fumes, or chemicals
• Not contagious
• Can occur when contact lenses are worn longer than recommended or not cleaned properly