Conjunctivitis, commonly known as pink eye, is one of the most widespread eye conditions affecting millions of people every year. This inflammation of the transparent membrane covering the eye can turn the white part of your eye pink or red, often accompanied by discharge and discomfort. While it usually resolves on its own, understanding the different types and causes can help you manage symptoms and prevent spreading it to others.

Epidemiology

Conjunctivitis stands out as one of the most common eye problems that brings people to seek medical care. In the United States alone, between three and six million people develop acute conjunctivitis each year^[1][6]. The condition places a substantial burden on the healthcare system, costing approximately $857 million annually in medical expenses^[4].

This eye condition does not discriminate by age or demographic group, affecting people of all ages and backgrounds. However, certain patterns emerge when examining who gets conjunctivitis most often. The condition is particularly common among school-aged children, where it spreads easily in group settings like classrooms and daycare centers^[11]. More than 80 percent of all acute cases in the United States are diagnosed by non-ophthalmologists, including primary care providers, pediatricians, and nurse practitioners, which reflects how frequently people encounter this condition in everyday healthcare settings^[4].

The frequency of conjunctivitis varies depending on the type. Viral conjunctivitis represents the most common overall cause of infectious conjunctivitis among all age groups^[13]. In contrast, bacterial causes tend to predominate in children, while viral infections are more common in adults^[6]. Allergic conjunctivitis affects a significant portion of the population, with studies suggesting that up to 40 percent of people experience it at some point, although only a small proportion of these individuals actually seek medical help^[13].

Causes

The root causes of conjunctivitis are diverse, ranging from infectious agents to environmental irritants. Healthcare providers typically divide conjunctivitis into two main categories: infectious and noninfectious^[2].

Infectious conjunctivitis develops when bacteria, viruses, fungi, or parasites invade the eye. Viral infections lead the pack as the most frequent culprit. The adenovirus family, which includes common cold viruses, causes most cases of viral conjunctivitis^[2]. Other viruses that can trigger this condition include measles, mumps, COVID-19, eye herpes, molluscum contagiosum, and hand, foot, and mouth disease^[2]. Sometimes viral conjunctivitis appears alongside symptoms of an upper respiratory infection, such as a sore throat, cough, or runny nose^[11].

Bacterial infections represent the second most common infectious cause. Several types of bacteria can trigger conjunctivitis, including those responsible for staph infections, strep throat, and certain types of meningitis^[2]. Bacterial conjunctivitis often produces thick, sticky discharge that may cause eyelids to stick together, especially after sleep^[11]. More concerning bacterial causes include sexually transmitted infections like chlamydia, gonorrhea, and syphilis, which can lead to pink eye and require more aggressive treatment^[2].

While less common, fungal and parasitic infections can also cause conjunctivitis, though they occur much less frequently than viral or bacterial infections^[2].

Noninfectious conjunctivitis develops from causes other than germs. Allergens like molds, pollen, or pet dander can irritate the conjunctiva, leading to allergic conjunctivitis^[2][5]. This form is especially common during allergy seasons when pollen counts are high. Various irritants and toxic substances can also trigger inflammation, including soap, cosmetics, dirt, smoke, and pool chlorine^[2]. Even contact lenses or contact lens solution can sometimes irritate eyes that usually tolerate them well.

Physical eye injuries that damage the conjunctiva can lead to inflammation. Additionally, some medications can cause pink eye as a side effect. In rare cases, underlying health conditions like autoimmune diseases, tumors, or eye cancers can irritate the eyes and cause symptoms similar to conjunctivitis^[2].

A severe form affecting newborn babies, called ophthalmia neonatorum, occurs when an infant is exposed to chlamydia or gonorrhea bacteria while passing through the birth canal during delivery^[5]. This serious condition requires immediate treatment to prevent permanent eye damage.

Risk Factors

Several factors increase the likelihood of developing conjunctivitis or experiencing more severe cases. Understanding these risk factors helps people take appropriate preventive measures.

Close contact with infected individuals represents one of the primary risk factors for infectious conjunctivitis. Both viral and bacterial pink eye spread easily from person to person through direct contact with eye discharge, contaminated surfaces, or respiratory droplets^[11]. Children in schools and daycare settings face particularly high exposure risk because of the close quarters and frequent hand-to-face contact typical in these environments.

People who wear contact lenses face an elevated risk of developing conjunctivitis, particularly bacterial forms that can be more severe^[4][7]. Contact lens wearers need to be especially vigilant about proper lens care and hygiene. Swimming in lakes or hot tubs while wearing contact lenses increases infection risk^[7]. Those who do not replace disposable lenses frequently enough or who wear soft contact lenses for extended periods may develop giant papillary conjunctivitis, a type of allergic response caused by the chronic presence of a foreign body in the eye^[5].

Poor hygiene practices significantly increase the risk of spreading or contracting infectious conjunctivitis. Touching the eyes with unwashed hands, sharing personal items like towels, washcloths, pillows, eye drops, or eye makeup can all transmit the infection^[16]. Using contaminated eye makeup or facial lotions, or sharing makeup with others, also raises infection risk^[5].

People with existing seasonal allergies are more prone to developing allergic conjunctivitis when they encounter allergens that trigger reactions in their eyes^[5]. Those with weakened immune systems face greater risk of developing complications from conjunctivitis and may need more aggressive treatment approaches^[10].

Newborn babies face unique risks if their mothers have untreated sexually transmitted infections at the time of delivery. Exposure to chlamydia or gonorrhea during birth can lead to ophthalmia neonatorum^[5].

Environmental factors also play a role. Exposure to irritants like smoke, air pollution, chlorine in swimming pools, or noxious chemicals increases the risk of chemical conjunctivitis^[5]. People who experience upper respiratory tract infections, such as colds, may find that the virus spreads to their eyes through the body’s connected mucous membranes^[5]. Forceful nose blowing can even push viruses from the respiratory system into the eyes through the tear ducts^[5].

Symptoms

The symptoms of conjunctivitis vary depending on what causes the condition, but certain signs appear consistently across most cases. The hallmark feature that gives this condition its common name is the distinctive pink or reddish appearance of the white part of the eye. This happens because inflammation causes the small blood vessels in the conjunctiva to swell and become more visible^[1][2].

Most people with conjunctivitis experience a gritty sensation in their eyes, as if sand or another small particle has become lodged there^[1][2]. This scratchy feeling can be quite bothersome and persistent. The affected eye or eyes may also feel itchy, though the intensity of itching varies by cause—allergic conjunctivitis typically produces severe itching, while viral and bacterial forms may cause less pronounced itching^[3].

Discharge from the eyes is another common symptom that helps distinguish between different types of conjunctivitis. Bacterial infections tend to produce thick, yellow, green, or white discharge that forms crusts on the eyelashes and eyelids^[2]. This crusty material often causes the eyelids to stick together, especially after sleeping, sometimes preventing the eyes from opening easily in the morning^[1]. Viral conjunctivitis usually produces a more watery discharge, while allergic conjunctivitis creates clear, watery tears^[5].

The eyes may appear puffy or swollen, with droopy eyelids^[2]. Many people experience increased tearing or excessive watering of the eyes^[1]. Paradoxically, some also report dryness alongside the watering. A burning sensation in the eyes frequently accompanies other symptoms^[2].

Photophobia, or sensitivity to light, makes it uncomfortable to be in bright areas or look at lights^[1][2]. This symptom is particularly common with viral conjunctivitis. Some people experience blurred vision that may come and go, though conjunctivitis rarely causes significant or lasting vision problems^[2]. Eye pain is usually mild when present, described more as discomfort than sharp pain^[2].

The specific pattern of symptoms often provides clues about the underlying cause. Bacterial conjunctivitis typically presents with mattering and adherence of the eyelids upon waking, lack of itching, and no history of previous conjunctivitis episodes^[13]. Viral infections frequently cause a sandy, gritty feeling like something is stuck in the eye, along with moderate to severe light sensitivity^[5]. Allergic conjunctivitis stands out for its intense itching, which is the most consistent sign of this type, often accompanied by clear watery discharge^[13][5].

Conjunctivitis can affect one eye initially and then spread to the other, or both eyes may be affected simultaneously^[2]. The condition’s appearance varies in severity—some cases produce dramatic-looking eyes with moderate redness and extreme discharge, while others present with milder symptoms^[5].

Prevention

Preventing conjunctivitis involves adopting good hygiene practices and taking specific precautions depending on the type of conjunctivitis you want to avoid. While there is no vaccine that prevents all types of pink eye, several strategies effectively reduce the risk of getting or spreading this condition^[16].

Hand washing stands as the single most important preventive measure. Wash your hands frequently with soap and water for at least 20 seconds, especially before and after touching your eyes or face^[3][16]. If soap and water are not available, use an alcohol-based hand sanitizer containing at least 60 percent alcohol. This simple habit dramatically reduces the transmission of infectious conjunctivitis.

Avoid touching or rubbing your eyes with your fingers whenever possible^[16]. This habit not only prevents introducing germs into your eyes but also stops you from spreading infection from one eye to the other. If you need to touch your eyes, always wash your hands first.

Never share personal items that come into contact with the eye area. This includes towels, washcloths, pillows, pillowcases, eye drops, eye makeup, makeup brushes, contact lenses, contact lens cases, and eyeglasses^[16]. Sharing these items creates a direct pathway for infection transmission.

For contact lens wearers, proper lens care is essential. Clean, store, and replace contact lenses exactly as instructed by your eye doctor^[16]. Wash your hands before putting in or removing lenses^[7]. Regularly clean and replace contact lens cases as recommended. Never swim in lakes or hot tubs while wearing contact lenses^[7]. If you develop conjunctivitis, stop wearing contacts immediately and do not resume use until the infection has completely cleared. Throw away disposable lenses and cases used while infected, and thoroughly clean reusable products before using them again^[16].

If you are around someone with pink eye, take extra precautions. Wash your hands thoroughly after any contact with the infected person or items they have used, such as after applying their eye drops or handling their bed linens^[16]. Do not share any personal items with them. If you develop conjunctivitis yourself, stay home from school or work, especially during the most contagious early days, and avoid swimming pools until the infection clears^[16].

To prevent allergic conjunctivitis, identify and avoid your specific allergens when possible. Remove allergens from your environment—for example, keep windows closed during high pollen days, use air filters, and minimize exposure to pet dander if you are allergic to animals^[10].

For preventing chemical conjunctivitis, minimize exposure to irritants like smoke, air pollution, and harsh chemicals. When swimming, consider wearing goggles to protect eyes from chlorine^[5].

Several vaccines protect against viral and bacterial diseases connected to conjunctivitis, including rubella, measles, chickenpox, shingles, pneumococcal disease, and Haemophilus influenzae type b^[16]. Keeping up to date with these vaccinations provides indirect protection against some forms of conjunctivitis.

For newborns, preventive measures start at birth. U.S. delivery rooms routinely apply antibiotic ointment to babies’ eyes as standard prophylactic treatment to prevent ophthalmia neonatorum^[5]. Pregnant women should be screened and treated for sexually transmitted infections before delivery to protect their babies.

Pathophysiology

Understanding how conjunctivitis develops requires looking at the normal structure and function of the conjunctiva, and then examining what changes occur during inflammation or infection.

The conjunctiva is a thin, translucent mucous membrane that serves as a protective and lubricating layer for the eye^[13]. It has two distinct parts: the bulbar conjunctiva, which covers the visible white part of the eyeball called the sclera, and the palpebral conjunctiva, which lines the inside surface of the eyelids^[1][13]. This transparent membrane allows light to pass through while keeping the eye surface moist and protected from foreign particles and microorganisms.

When conjunctivitis develops, inflammation causes the small blood vessels within the conjunctiva to become swollen and irritated^[1]. As these tiny vessels dilate and engorge with blood, they become much more visible through the transparent membrane. This vascular engorgement creates the characteristic reddish or pink appearance that gives the condition its common name. The inflammation also triggers increased blood flow to the area as part of the body’s immune response.

In infectious conjunctivitis, the pathophysiology begins when bacteria, viruses, or other microorganisms breach the eye’s natural defenses and invade the conjunctival tissue. The body recognizes these foreign invaders and mounts an immune response. White blood cells rush to the area to fight the infection, contributing to the swelling and redness. The conjunctiva responds by producing increased discharge as it attempts to flush out the infectious agents. The type and consistency of discharge varies with the infecting organism—bacteria trigger production of thick, purulent material, while viruses typically cause watery secretions.

In allergic conjunctivitis, the process differs significantly. When allergens contact the conjunctiva, they trigger an immune system overreaction in people who are sensitized to those substances. The body releases histamine and other inflammatory chemicals that cause blood vessels to dilate and become leaky. This leads to swelling, redness, and the intense itching characteristic of allergic reactions. The eyes water excessively as tear production increases in response to the perceived threat.

Chemical or irritant conjunctivitis develops through direct toxic damage to the conjunctival tissue. Irritating substances cause physical and chemical injury to the cells, triggering an inflammatory response even without infection. The conjunctiva swells and reddens as the body attempts to heal the damage and remove the irritant through increased tear production.

The inflammatory process in conjunctivitis involves complex biochemical changes. Inflammatory mediators released by immune cells increase the permeability of blood vessel walls, allowing fluid and proteins to leak into surrounding tissues. This creates the edema or swelling seen in conjunctivitis. The same inflammatory chemicals stimulate nerve endings, causing the burning, itching, and gritty sensations that accompany the condition.

Throughout these processes, the normal mechanical functions of the conjunctiva become disrupted. The smooth gliding motion of the eyelid over the eye surface becomes impaired by swelling and excess discharge. The protective tear film that normally coats and lubricates the eye may be compromised, leading to feelings of dryness despite increased tearing. In severe cases, the inflammation can extend beyond the conjunctiva to affect adjacent structures, though the cornea—the clear front surface of the eye—usually remains unaffected, which is why vision typically remains normal^[1].

The duration of these pathophysiological changes depends on the cause and the body’s ability to resolve the inflammation. Viral conjunctivitis must run its course as the immune system clears the virus, typically taking two to three weeks^[8]. Bacterial infections may resolve faster, especially with antibiotic treatment. Allergic conjunctivitis persists as long as allergen exposure continues, but symptoms resolve quickly once the allergen is removed from the environment.