What Usually Causes Conjunctivitis?

Medically Reviewed on 10/19/2022
What Usually Causes Conjunctivitis
Conjunctivitis is usually caused by a viral or bacterial infection

Conjunctivitis or pink eye is usually caused by a viral or bacterial infection, an allergic reaction, or an irritant such as a foreign object or chemical.

Conjunctivitis occurs when there is inflammation of the conjunctiva—the thin, transparent membrane (conjunctiva) that covers the white surface of the eye (sclera) that lines the eyelid. Blood vessels in the sclera grow and become more noticeable when the conjunctiva is inflamed, giving the eye its red appearance.

Your doctor can tell you the cause of your conjunctivitis, whether it's contagious, and what treatment is best for your specific type.

What is viral or bacterial conjunctivitis?

Most cases of conjunctivitis are caused by viruses or bacteria.

  • Viral conjunctivitis:
    • Most common type of conjunctivitis
    • Highly contagious and frequently spreads in schools and crowded places
    • Symptoms include red, burning eyes with a watery discharge
    • Typically caused by the same virus as the common cold 
  • Bacterial conjunctivitis:
    • Extremely contagious
    • Symptoms include red, sore eyes with or without pus or discharge
    • Sometimes caused by the same bacteria that cause strep throat
  • Sexually transmitted infections:
    • Gonorrhea can cause a rare but potentially dangerous form of bacterial conjunctivitis that if left untreated can cause vision loss
    • Adults can get conjunctivitis from chlamydia

Conjunctivitis can spread easily from person to person. If you have had close contact with someone who has viral or bacterial conjunctivitis, your risk increases.

Therefore, it's crucial to thoroughly wash your hands after interacting with anyone who has conjunctivitis. Additionally, you should refrain from sharing towels or pillowcases with anyone who has been infected.

Risk factors of conjunctivitis include:

  • Age (children and older people are more likely to contract it, presumably because of increased exposure to diseases and weakened immune systems)
  • Recent upper respiratory infections such as the cold or flu
  • Diabetes or another illness that compromises your immune system
  • Use of corticosteroids which can impair your immunological function.
  • Blepharitis (an infection of the eyelid rims) 

What is allergic conjunctivitis?

When an allergen (pollen, car fumes, cigarette smoke, animals, chlorine, or other environmental factors) comes into contact with your eyes, it can cause allergic conjunctivitis. Allergic pink eye causes red, itchy, watery eyes. and puffy eyelids.

The following are the main types of allergic conjunctivitis:

Seasonal and perennial conjunctivitis

Common causes of seasonal allergic conjunctivitis include:

  • Grass, tree, or flower pollen
  • Dust mites
  • Dead animal skin

These types frequently co-occur with allergic rhinitis and are more prevalent in people with other allergies or conditions such as asthma.

Contact dermatoconjunctivitis

Common causes of contact dermatoconjunctivitis include:

  • Eye drops
  • Makeup 
  • Chemicals

Giant papillary conjunctivitis

Causes of giant papillary conjunctivitis include:

  • Contact lenses (according to estimates, 1% of people who wear hard contact lenses and 1%-5% of people who wear soft contact lenses experience giant papillary conjunctivitis)
  • Stitches used during eye surgery
  • Surgery to place an artificial eye

What is irritant conjunctivitis?

Causes of irritant conjunctivitis vary widely and may include:

Symptoms of irritant conjunctivitis usually resolve on their own within 24 hours.


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How can I tell if I have pink eye?

Typical signs of pink eye include:

  • Dry eyes
  • Aching eyes (usually occurs with bacterial conjunctivitis)
  • Swollen eyes
  • Swollen eyelids
  • Blurred vision
  • Increased sensitivity to light
  • Feeling like a foreign body is in your eye or the need to rub your eye
  • Pus, mucus, or thick yellow discharge coming from your eye which can cause your eyelashes to become matted

How is conjunctivitis diagnosed?

Your ophthalmologist can diagnose conjunctivitis simply by examining your eye. They can usually tell whether the inflammation is caused by a viral or bacterial infection based on your symptoms. 

To confirm a diagnosis, the following tests may be performed:

  • Medical history: Many causes of conjunctivitis can be ruled out by assessing your symptoms and how they developed.
  • Slit lamp examination: Your doctor may use an instrument that combines a microscope and a high-energy light beam to examine your eye.
  • Visual acuity test: This test is used by doctors to determine whether conjunctivitis has affected your vision and assesses your ability to read letters or symbols from 20 feet away while covering one eye at a time.
  • Eye culture: A pathologist can tell you whether your conjunctivitis is caused by bacteria or viruses. This assists your doctor in determining the most effective treatment option.

How is conjunctivitis treated?

When determining which medication is best for you, your ophthalmologist will consider the cause of conjunctivitis and your symptoms. Treatment may include:

Conjunctivitis should be treated immediately and the following precautions should be taken:

  • Avoid putting touching your eyes.
  • Wash your hands thoroughly both before and after using eye medications.
  • Avoid sharing towels, washcloths, cosmetics, or eye drops.
  • Until the disease has been treated, young children should be kept out of school and swimming pools.

Treatment for viral or bacterial conjunctivitis

Typically, antibiotic eye drops and/or ointments are used to treat bacterial conjunctivitis. 

Antibiotics are ineffective in treating viral conjunctivitis. However, some antibiotics may be administered to stop the emergence of secondary bacterial infections. As viral conjunctivitis progresses, artificial tears and warm compresses may help keep the eye comfortable.

Treatment for allergic conjunctivitis

Conjunctivitis that is allergic or chemical in nature must first be treated for the underlying allergy or irritation. For instance, if chlorinated water bothers your eyes, wear goggles when swimming. Prescription and over-the-counter eye solutions are available to ease the discomfort if these efforts prove unsuccessful.

How do you prevent the spread of pink eye?

Conjunctivitis can be stopped from spreading by practicing good hygiene. Tips to prevent the spread of the disease include the following:

  • Keep your hands out of your eyes.
  • Frequently wash your hands.
  • Carefully clean and replace contact lenses regularly.
  • Change and wash pillowcases frequently.
  • Replace eye makeup frequently and avoid sharing it with others.
  • Avoid sharing handkerchiefs or towels.
Medically Reviewed on 10/19/2022
Image Source: iStock image

Conjunctivitis: What Is Pink Eye? https://www.aao.org/eye-health/diseases/pink-eye-conjunctivitis

Pink Eye (Conjunctivitis) https://my.clevelandclinic.org/health/diseases/8614-pink-eye

Pinkeye (Conjunctivitis) https://kidshealth.org/en/parents/conjunctivitis.html

Conjunctivitis (pink eye) https://www.aoa.org/healthy-eyes/eye-and-vision-conditions/conjunctivitis?sso=y