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 [1]
  • 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 [2]
  • More common in kids than adults
  • Observed more frequently December through April

Allergic Conjunctivitis

girl with itchy eyes sitting among flowers
  • 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

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Footnotes

1Azari AA, Barney NP. Conjunctivitis: A systematic review of diagnosis and treatmentExternal. JAMA. 2013;310(16):1721–9.
2American Academy of Pediatrics. Management and prevention of infectious diseases: Exclusion and return to care.External In: Kimberlin DW, Brady MT, Jackson MA, Long SS, eds. Red Book: 2018 Report of the Committee on Infectious Diseases. 31st ed. Elk Grove Village, IL: American Academy of Pediatrics. 2018;125–36.