What Causes Conjunctivitis?

Conjunctivitis has a number of potential causes. It can be caused by viral or bacterial infection, allergies, and chemical or mechanical irritation.

Close up of the chronic conjunctivitis with concretion during eye examination. Image Credit: ARZTSAMUI / Shutterstock
Close up of the chronic conjunctivitis with concretion during eye examination. Image Credit: ARZTSAMUI / Shutterstock

Viral Conjunctivitis

Viral conjunctivitis, also known as pinkeye, is usually caused by adenovirus. Herpes simplex virus, varicella-zoster virus, picornavirus, poxvirus, and human immunodeficiency virus are other viruses that can cause conjunctivitis. Conjunctivitis can rarely occur in systemic infection with influenza virus, Epstein-Barr virus, measles, mumps, and rubella.

Transmission of viral conjunctivitis occurs through hand to hand contact, droplets, fomites, or swimming pools. It is highly contagious for 10 to 12 days from onset and resolves within two to four weeks.

Bacterial Conjunctivitis

Bacterial conjunctivitis comprises about 30% of conjunctivitis cases. The most common bacteria that cause this disease are Staphylococcus and Streptococcus species. Other possible infection agents are Corynebacterium, Haemophilus, Pseudomonas, and Moraxella species.

Rarely, more dangerous organisms like Chlamydia trachomatis and Neisseria gonorrhoeae can cause conjunctivitis. Gram-positive infections like Staphylococcus aureus and Streptococcus pneumoniae are relatively less severe and self-limiting. Gram-negative infections like Pseudomonas, Serratia marcescens, Haemophilus influenzae, Moraxella, C. trachomatis, and N. gonorrhoeae can become severe with potential complications leading to blindness.

Allergic Conjunctivitis

Allergic conjunctivitis falls into several subcategories. Seasonal allergic conjunctivitis is caused by seasonal allergens such as pollen, mold, and weeds. Perennial allergic conjunctivitis is caused by allergens that are present at all times regardless of season, like dust mites and animal dander. Atopic keratoconjunctivitis, giant papillary conjunctivitis, and limbal and vernal keratoconjunctivitis are rarer forms of allergic conjunctivitis.

A typical allergic reaction, known as Type I hypersensitivity, occurs when a person is exposed to the antigen. The body produces an immune response mediated by the molecule immunoglobulin E (IgE). The IgE response triggers a release of inflammatory chemicals including histamines, prostaglandins, and other molecules.

Seasonal allergic conjunctivitis typically lasts for the duration of the life cycle of the offending plant. In perennial allergic conjunctivitis, symptoms continue year round.  Vernal keratoconjunctivitis is associated with atopic illness like asthma, eczema, and allergic rhinitis. Atopic keratoconjunctivitis is similar to atopic dermatitis. It is also a Type I hypersensitivity disorder.

Giant papillary conjunctivitis is an inflammatory disorder of the conjunctiva characterized by papillae larger than 0.3 mm in diameter. It is commonly diagnosed in contact lens wearers. The mechanism is believed to be either mechanical irritation or antigenic stimulus of the conjunctiva of the upper eyelid. That leads to changes in the tissue, conjunctivitis, and the formation of large papillae. Dirty contact lenses may contribute to the development of giant papillary conjunctivitis.

Irritant Conjunctivitis

Conjunctivitis can be caused by a chemical irritant or a mechanical insult like a foreign body. Some common irritants that may lead to conjunctivitis are dry eye, contact with acid or alkali solutions, a foreign body, abrasions, and blunt trauma. Severity can range from minor irritation to trauma. Some common irritants are dry eye, acid or alkali solutions, foreign bodies, and abrasions to the eye.

Pollution, smoke, and contact lenses can also cause irritation leading to conjunctivitis. Removing the source of irritation, by flushing, for example, or discontinuing the use of the product, usually resolves conjunctivitis within two to three days.


  1. http://emedicine.medscape.com/article/1191370-overview#a1
  2. http://emedicine.medscape.com/article/1191730-overview
  3. http://emedicine.medscape.com/article/1191467-overview#a4
  4. https://www.dermnetnz.org/topics/irritant-or-traumatic-conjunctivitis
  5. http://emedicine.medscape.com/article/1191641-overview#a7

Further Reading

Last Updated: Feb 26, 2019

Dr. Catherine Shaffer

Written by

Dr. Catherine Shaffer

Catherine Shaffer is a freelance science and health writer from Michigan. She has written for a wide variety of trade and consumer publications on life sciences topics, particularly in the area of drug discovery and development. She holds a Ph.D. in Biological Chemistry and began her career as a laboratory researcher before transitioning to science writing. She also writes and publishes fiction, and in her free time enjoys yoga, biking, and taking care of her pets.


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