Why is pink eye so contagious?

Any parent of young children dreads hearing the words "pink eye." This common childhood infection, also called conjunctivitis, can spread rapidly, and once the little ones bring it home, it can be passed easily to other members of the household. 

But why is pink eye so contagious?

One factor is that conjunctivitis, or inflammation of the conjunctiva — a thin, clear mucous layer that covers the inner surface of the eyelid and the white part of the eyeball — is often caused by very infectious bacteria and viruses, Dr. Paramdeep Bilkhu, an optometrist and clinical adviser at the College of Optometrists in London, told Live Science by email. 

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Although allergies or irritants such as a loose eyelash can cause pink eye, most cases come from viruses and bacteria. Adenoviruses, a highly contagious family of pathogens that also cause cold- and flu-like symptoms, account for over 75% of cases of infective conjunctivitis, Bilkhu said. 

Adenoviruses are also extremely hardy. For example, these viruses may survive disinfectants, such as 70% rubbing-alcohol (such as common hand sanitizers) and 3% hydrogen peroxide (such as Lysol and other household cleaners) solutions, according to a 2020 systematic review published in the Journal of Ophthalmic & Vision Research.

What's more, a person infected with an adenovirus can be contagious for up to two weeks from the time they are infected. Lastly, no treatments target adenoviruses, the review authors noted.

"While in some cases of bacterial infective conjunctivitis antibiotics are required to help bring about resolution, they will not work where the cause is viral," Bilkhu said. "There are no antiviral agents available to treat the viruses typically implicated in infective conjunctivitis."

Related: What are eyes made of?

The bacteria and viruses behind pink eye spread as a direct result of the symptoms they cause, such as red, watery eyes; sticky discharge; and a burning or itching sensation. People with pink eye have an intense urge to rub the affected eyes. From those infected surfaces, the microbes can go from someone's hands to surfaces and personal items, according to the Centers for Disease Control and Prevention (CDC). 

Viruses that cause pink eye also may be transmitted through droplets expelled when someone coughs or sneezes. That is because they are able to spread along the mucous membranes that connect the respiratory tract, tear ducts and conjunctiva, according to the American Optometric Association. "Many cases of viral conjunctivitis occur following a prior or close contact with someone who has an upper respiratory tract infection, such as a cold or flu," Bilkhu said. 

Because young children spend a lot of time in close contact with each other and do not tend to practice good hygiene, they are particularly susceptible to pink eye, according to the American Academy of Ophthalmology. 

Most cases of infective conjunctivitis are mild and resolve within two weeks without any long-term complications. "During this period, the key is to ensure careful hygiene measures to prevent spread to the fellow eye and to others," Bilkhu said. "This includes washing hands before and after touching the eyes, appropriately disinfecting surfaces after touching them, and changing bedding and pillow cases each night."

This article is for informational purposes only and is not meant to offer medical advice. 

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