Conjunctivitis (pink eye) is usually caused by viral infections. Bacterial infections are also a relatively common cause. Less often, it’s caused by allergies, irritants, or underlying medical conditions.

Conjunctivitis is inflammation of the transparent membrane that covers the eyeball and the inside of the eyelids, the conjunctiva. The condition is popularly known as “pink eye” because it often makes the white part of the eye look reddish or pink.

Most of the time, conjunctivitis resolves without complications, either by itself or with home remedies. Infectious causes can make the condition contagious.

The causes of conjunctivitis may influence symptoms. For example, some autoimmune conditions are more likely to cause symptoms of conjunctivitis in one eye only.

Pink eye is the most common cause of eye redness and discharge. The underlying causes of conjunctivitis can be divided into infectious and noninfectious.

Infectious conjunctivitis

About 80% of cases of acute conjunctivitis are caused by viral infections. The most common type of virus that causes conjunctivitis is adenovirus, making up between 65% and 90% of viral cases.

Other potential viral infections that lead to inflammation of the conjunctiva include:

Characteristic signs of viral conjunctivitis are often sudden and may include:

Some symptoms that may help differentiate viral conjunctivitis from other types include:

Only a healthcare professional can provide an accurate diagnosis. If you experience any of these symptoms, it’s a good idea to consult a doctor.

Bacterial and other infectious causes

Bacterial infections are more common in children. Staph infections are the most common in adults, especially Staphylococcus aureus andStreptococcus pneumoniae, followed by Haemophilus influenza.

Bacterial conjunctivitis can cause similar eye symptoms to viral conjunctivitis, but yellow or green discharge is more common in bacterial infections.

With bacterial infections, your eye may feel itchy, and your eyelid may become swollen.

Other types of infections can also cause conjunctivitis, but they’re much rarer. They can include fungi and parasites.

Noninfectious causes

Conjunctivitis can also develop from irritants and allergens. Potential noninfectious causes of conjunctivitis may include:

  • allergens (substances that trigger an allergic reaction) such as:
    • air pollution
    • dust
    • pet dander
  • toxins
  • various other irritants

Conjunctivitis that results from irritants or allergens often causes symptoms shortly after direct contact with your eye. You’re also unlikely to develop typical symptoms of an infection, such as fever or swollen lymph nodes.

Rarely, allergic conjunctivitis can jeopardize your vision. If you experience any eye symptoms for more than 1 day, you should seriously consider consulting a healthcare professional.

Vitamin D deficiency doesn’t cause pink eye, but it may make people more prone to infections that may cause it.

In a 2024 study, researchers found evidence that restoring levels of vitamin D may lead to better outcomes in people with severe or persistent conjunctivitis symptoms.

Conjunctivitis can be a sign of some medical conditions that cause inflammation throughout your body or specifically in the eye.

Conditions that have been linked to conjunctivitis symptoms or eye irritation include:

If you develop conjunctivitis related to an inflammatory or autoimmune condition, you’re likely to also develop other symptoms of this condition. For instance, lupus erythematosus can cause additional symptoms such as:

Only a healthcare professional can perform a comprehensive assessment of your symptoms and provide a diagnosis.

Stress doesn’t cause conjunctivitis, but if you live with chronic stress, you may be more prone to develop infections, including those that cause conjunctivitis. Chronic stress is linked to lower immunity.

Stress may also indirectly lead to red eyes if you’re sleep deprived or spend a lot of time in front of screens. High levels of anxiety may also increase adrenaline and cortisol production, which can reduce tear production, lead to insomnia, and constrict blood vessels in the eyes. These are all risk factors for conjunctivitis by way of different mechanisms, like decreased immunity.

If you have had viral conjunctivitis, you may be more likely to develop it again. Recurrent infections in the eyes may also be a sign that your immune system is compromised. If this is the case for you, it’s recommended that you visit a healthcare professional for a comprehensive health exam.

Conjunctivitis or pink eye has many causes, including viruses, bacteria, allergens, and irritants. Symptoms may vary depending on the case, but they usually include eye redness, itchiness, and some type of discharge.

If you have symptoms in your eyes that persist for days or become recurrent, it’s a good idea to seek medical care as soon as possible to prevent complications.