Causes and Risk Factors of Pink Eye

Conjunctivitis, more commonly known as pink eye, affects as many as six million people each year in the United States. It is important to understand the different causes and risk factors of conjunctivitis in order to know how to properly manage your symptoms and to prevent recurrence.

Overall, conjunctivitis falls into two main categories: infectious and non-infectious. Infectious conjunctivitis is caused by bacteria, viruses, or fungi while non-infectious causes include allergies, chemical irritants, and foreign bodies.

pink eye causes
Illustration by Joshua Seong. © Verywell, 2018.

Causes of Viral Conjunctivitis

Viruses account for 80% of all cases of conjunctivitis. The most common symptoms are eye redness and watery discharge.

Common viruses include:

Up to 90% of viral conjunctivitis cases are caused by adenovirus. Around 5% are caused by the herpes simplex virus.

Treatment is rarely needed, as viral infections usually resolve on their own. However, two exceptions require referral to an ophthalmologist.

Herpes infections

Not only can herpes infections like herpes simplex and varicella-zoster inflame the conjunctiva but they can sometimes cause corneal ulcers or scarring of the cornea that could affect your vision. There are tests your healthcare provider can perform to find out if you have a herpes infection.

Epidemic keratoconjunctivitis

One extremely contagious form of viral conjunctivitis, epidemic keratoconjunctivitis (EKC), is caused by specific serotypes of adenovirus. EKC inflames both the cornea and the conjunctiva, potentially causing changes to your vision. In addition to watery discharge, you may feel like there is a foreign body in your eye.

Causes of Bacterial Conjunctivitis

Bacterial causes of conjunctivitis are far less common. Ocular discharge is typically thick and purulent as opposed to the watery discharge often seen with viral infections.

Bacterial conjunctivitis requires treatment with the appropriate antibiotics to decrease the spread of infection.

Common Causes

  • Staphylococcus aureus
  • Streptococcus pneumoniae
  • Haemophilus influenzae
  • Moraxella catarrhalis

S. aureus is the most common bacteria found in adults while children are more likely to be infected by the other bacteria listed. In most cases, these bacteria are easily treated.

The one exception is the methicillin-resistant S. aureus (MRSA). MRSA infection will require an ophthalmology consultation and treatment with specific antibiotics.

Rare Causes

There are two aggressive bacterial infections that warrant further attention. While they are not common, they could increase the risk of vision loss. A formal ophthalmology evaluation is advised.

Chlamydia trachomatis and Neisseria gonorrhoeae are the bacteria responsible for the sexually-transmitted infections you know more commonly as chlamydia and gonorrhea. While we do not often think of these sorts of infections as getting into the eyes, it happens. For example, someone can rub their eyes after touching infected body fluids or secretions.

The population at greatest risk for eye infections caused by STDs is newborns. If the mother is infected at the time of delivery, transmission occurs as the baby exits the birth canal.

Chlamydia and gonorrhea do not always cause symptoms, meaning the mother may or may not know she is infected. It is for this reason that the standard of care at delivery is to treat all newborns with antibiotic ointment.

Allergic Conjunctivitis

People who have seasonal allergies, asthma, and eczema are at increased risk of developing allergic conjunctivitis. What distinguishes allergic conjunctivitis from the infectious types is the itching. Like viral conjunctivitis, ocular discharge tends to be watery.

The remaining cases may be caused by other allergic exposures or chronic allergies. In very rare cases, the inflammation caused by the allergic response extends to the cornea, leading to atopic keratoconjunctivitis (AKC). As with any keratitis, there is an increased risk of vision impairment if AKC is left untreated.

Seasonal allergies account for 90% of all allergic conjunctivitis cases.

Other Common Causes

Other forms of conjunctivitis are usually short-lived and may be caused by the following.

Chemical Exposures

If a chemical gets into your eye, it can cause irritation and redness. Chlorinated pool water is a common example.

It is also possible that a toxic chemical gets splashed into your eye. Eye irrigation may remove the offending agent but could also cause eye redness in and of itself. Redness after irrigation usually improves within a day. 

Foreign Bodies

A foreign body in the eye, even an eyelash, can trigger eye redness and inflammation for up to a day after it is expelled. Eye irrigation to remove that foreign body can add to that irritation.

More concerning is when a foreign body exposure is more chronic in nature. This is where ​giant papillary conjunctivitis (GPC) comes into play. GPC occurs when the eyelid rubs repeatedly against a foreign body like a contact lens or a surgical stitch. An immune reaction is triggered that leads to local inflammation.

Not only will people with GPC get itchy watery eyes, they often describe a gritty sensation. The eyelid also thickens and forms small bumps on the underside of the eyelid that help to establish the diagnosis. GPC is more likely to occur if debris has collected on your contact lens.

GPC is 10 times more common in users of soft contact lenses than hard contacts.

Still, it is not very common, affecting only about 5 percent of soft contact users.

Lifestyle Factors

You may not be able to control whether someone near you gets conjunctivitis but you can take steps to decrease the risks to yourself.

Contact Lenses

Wearing contact lenses could increase your risk for conjunctivitis in a number of ways. The contact lens cleaning solution could become infected with bacteria or the solution itself could be chemically irritating to the eye. The contact lens itself may not fit properly or deposits may build up on the lenses after long-term use or with improper cleaning.

If you use contact lenses, take care to clean them properly and to see an eye doctor if you have discomfort with use.

Dry Eyes

People with dry eye syndrome are more prone to develop pink eye. You may consider using hydrating eye drops or seeking an evaluation with an eye doctor to see if other treatments are indicated.


Poor hygiene makes it more likely that you could spread infection from one eye to the other or it can spread from one person to another. Frequent handwashing is key.

Also avoid touching or rubbing your eyes and sharing anything that could come into contact with your eyes, i.e., contact lenses, eye makeup, eyeglasses, pillows, or towels.

Frequently Asked Questions

  • Can COVID-19 cause pink eye?

    Yes. The virus can infect the eye if you touch your eye after touching an infected surface, or it can enter the eye from the nasal passages after you’ve been infected. In rare cases, pink eye may be the only symptom of COVID-19.

  • What is an eye cold?

    “Eye cold” is sometimes used to refer to viral conjunctivitis (pink eye). It’s usually caused by the common adenovirus, which gives you mild cold or flu symptoms. This type of eye infection will often clear up on its own with time.

  • How do you get an MRSA infection in the eye?

    This uncommon type of bacterial infection occurs if you are exposed to MRSA during an injury, surgery, or an illness. If you already have an MRSA infection, the eye could become affected if the bacteria travel to the eye via the nasal passages.

11 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Azari AA, Barney NP. Conjunctivitis: a systematic review of diagnosis and treatment. JAMA. 2013;310(16):1721-1730. doi:10.1001/jama.2013.280318

  2. Kalogeropoulos CD, Bassukas ID, Moschos MM, Tabbara KF. Eye and Periocular Skin Involvement in Herpes Zoster InfectionMed Hypothesis Discov Innov Ophthalmol. 4(4):142-156.

  3. Meyer-Rüsenberg B, Loderstädt U, Richard G, Kaulfers PM, Gesser C. Epidemic keratoconjunctivitis: the current situation and recommendations for prevention and treatmentDtsch Arztebl Int. 2011;108(27):475-480. doi:10.3238/arztebl.2011.0475

  4. Epling J. Bacterial conjunctivitisBMJ Clin Evid. 0704.

  5. Silvester A, Neal T, Czanner G, Briggs M, Harding S, Kaye S. Adult bacterial conjunctivitis: resistance patterns over 12 years in patients attending a large primary eye care centre in the UKBMJ Open Ophthalmol. 2016;1(1):e000006. doi:10.1136/bmjophth-2016-000006

  6. La Rosa M, Lionetti E, Reibaldi M, et al. Allergic conjunctivitis: a comprehensive review of the literatureItal J Pediatr. 2013;39:18. doi:10.1186/1824-7288-39-18

  7. Ahmad SS. Water related ocular diseasesSaudi J Ophthalmol. 2018;32(3):227-233. doi:10.1016/j.sjopt.2017.10.009

  8. Boyd K. Giant Papillary Conjunctivitis. American Academy of Ophthalmology.

  9. American Academy of Ophthalmology. COVID conjunctivitis.

  10. Azari AA, Barney NP. Conjunctivitis: a systematic review of diagnosis and treatment. JAMA. 2013;310(16):1721. doi:10.1001%2Fjama.2013.280318

  11. American Academy of Ophthalmology. What is the treatment of an MRSA infection of the eye due to pink eye?

Additional Reading

By Tanya Feke, MD
Tanya Feke, MD, is a board-certified family physician, patient advocate and best-selling author of "Medicare Essentials: A Physician Insider Explains the Fine Print."