Viral or bacterial conjunctivitis: what’s the difference?

Conjunctivitis is inflammation of the conjunctiva, the thin, clear layer that covers the white of your eye and lines the inside of your eyelids. The NHS describes it as an eye condition caused by infection or allergies. It usually affects both eyes and makes them red, gritty, itchy, watery or sticky with pus.

There are three common types:

• Viral: caused by a virus, usually one of the viruses behind colds and flu.
• Bacterial: caused by bacteria. The College of Optometrists says it occurs more often in children and older people.
• Allergic: a reaction to something like pollen, house dust mites or animals. It isn’t an infection, so you can’t pass it on.

Viral and bacterial conjunctivitis are together known as infective conjunctivitis. UK figures are limited. In one trial in 30 general practices in southern England, bacteria were found in half of the eye swabs tested. A review in the American medical journal JAMA says viruses cause up to 80% of all cases of acute conjunctivitis, and that a group of viruses called adenoviruses is behind 65% to 90% of the viral cases. It says bacteria are the second most common cause of infective conjunctivitis, but cause most cases in children.

The table shows what each type is more likely to look like. Treat it as a guide, not a diagnosis.

Either type can come with flu-like symptoms, such as a sore throat, a high temperature and aching muscles, according to Moorfields Eye Hospital. The JAMA review says swollen glands are seen in up to half of viral cases, and are more common with viral conjunctivitis than with bacterial.

Not reliably. The signs in the table point one way or the other, but research shows they aren’t enough to be sure.

• Discharge tells you less than you’d think. A study in The BMJ looked at 184 adults who went to Dutch health centres with a red eye and either discharge or glued eyelids. It found that the type of discharge told doctors almost nothing about whether bacteria were the cause. Only about a third of the eyes tested (32%) had bacteria in them.
• Three clues did help. In the same study, eyelids glued together in the early morning made bacteria more likely. Itching, and having had conjunctivitis before, made bacteria less likely.
• Symptoms overlap. The JAMA review describes a study of people with confirmed bacterial conjunctivitis in which 58% had itching and 35% had a watery discharge or none at all.
• Doctors find it hard too. An analysis of three trials in UK and Dutch general practice, published in the British Journal of General Practice, said that telling a viral cause from a bacterial one is difficult from symptoms and signs.

A swab can be tested in a laboratory, but the same analysis notes that it’s generally impractical to wait for the result before deciding on treatment. For most people it makes little difference, because most cases get better whichever type they are.

Most infective conjunctivitis clears up without treatment within a couple of weeks. That’s the NHS’s summary for conjunctivitis in general. In more detail:

• Bacterial: the College of Optometrists says it often clears up in five to seven days without treatment. In trials, about 56% of people given dummy eye drops were better within 4 to 9 days.
• Viral: Moorfields Eye Hospital says it can last for two to three weeks, which is how long your immune system may need to clear the virus.

See a doctor if your symptoms haven’t cleared up within 7 days. The NHS says conjunctivitis caused by an STI takes longer to get better, which is one reason to get a lingering red eye checked.

Viral and bacterial conjunctivitis are both contagious. Allergic conjunctivitis isn’t. Moorfields Eye Hospital says both infective types spread very easily, through droplets from coughs and sneezes and through contact with tissues, flannels, towels and pillowcases. The College of Optometrists describes viral conjunctivitis as highly infectious, and bacterial conjunctivitis as contagious.

The NHS doesn’t give a number of days. It says conjunctivitis that produces sticky pus is contagious, and that red, gritty eyes usually are too. The JAMA review estimates that viral conjunctivitis can be passed on for 10 to 14 days and bacterial conjunctivitis for 2 to 7 days.

To avoid passing it on, the NHS suggests:

• washing your hands regularly with warm soapy water
• washing pillowcases and face cloths in hot water and detergent
• covering your mouth and nose when you sneeze, and putting used tissues in the bin
• not sharing towels or pillows
• not rubbing your eyes

The NHS says you don’t need to stay away from work or school unless you or your child feel very unwell. UK Health Security Agency guidance for schools and nurseries says people with conjunctivitis can attend as usual.

Most cases need no medical treatment, only simple care at home while the infection clears. The NHS suggests:

• cleaning your eyelids with water that has been boiled and left to cool, using a clean cotton wool pad for each eye
• holding a cold flannel on your eyes for a few minutes to cool them
• leaving contact lenses out until your eyes are better

A pharmacist can give advice and suggest eye drops or antihistamines to help with your symptoms.

Viral conjunctivitis

Antibiotics don’t work on viruses, and the College of Optometrists says there’s no effective antiviral drug for ordinary viral conjunctivitis. Cool compresses and lubricating eye drops can ease the discomfort while your body clears the virus. Moorfields Eye Hospital says the drops may reduce grittiness but won’t shorten the illness.

Bacterial conjunctivitis

Antibiotic eye drops or ointment can speed things up, but only a little. In the analysis of three general practice trials, which took people with infective conjunctivitis of either type, 80% of those given antibiotics were cured by day 7, compared with 74% of those who weren’t. A 2023 Cochrane review of 21 trials found the same pattern: 68% of people given antibiotic drops were better within 4 to 9 days, against 56% of those given dummy drops.

One antibiotic, chloramphenicol, can be bought from pharmacies as eye drops or ointment for most adults and children aged 2 and over. Children under 2 need a prescription.

Allergic conjunctivitis

This is treated with anti-allergy eye drops or antihistamines, and antibiotics don’t help. It causes redness, watering and itching, is more likely to affect both eyes, and often comes with hay fever or an allergy to animals.

Get urgent medical help if a red eye comes with pain, sensitivity to light or a change to your sight. On its conjunctivitis page, the NHS lists these as possible signs of a more serious eye problem:

• pain in your eyes
• sensitivity to light
• changes in your vision, like wavy lines or flashing
• very red eyes, in one eye or both
• a baby less than 30 days old with red, sticky eyes

A red eye is also urgent if you wear contact lenses, because it could be an infection of the cornea, the clear window at the front of the eye.

In an emergency, call 999, for example if it hurts to look at light, you lose some of your sight, you have a severe headache and feel sick, your eye is very dark red, or one pupil is bigger than the other.

Not often. But Chlamydia and Gonorrhoea can both infect the eye, and Gonorrhoea in the eye is an emergency. The NHS says some STIs can cause conjunctivitis, and that this type takes longer to get better. The bacteria reach the eye in infected semen or vaginal fluid, including on your own fingers.

Chlamydia

UK guidelines say Chlamydial conjunctivitis more typically affects one eye, can come on quickly or linger, and is hard to tell from viral conjunctivitis without a lab test. Up to 8 in 10 people who have it also have Chlamydia in their genitals, often without knowing. That’s why it’s treated with antibiotic tablets, usually doxycycline for 7 days, and not with eye drops alone. Read more in “Can you get Chlamydia in your eyes?”

Gonorrhoea

Gonorrhoea in the eye usually causes a red eye with a lot of pus. UK guidelines warn that the bacteria can get through the surface of the cornea and cause ulcers that threaten your sight. So it’s treated straight away, with an antibiotic injection and antibiotic eye drops, and with an urgent referral to an eye specialist. Get urgent medical help if you have Gonorrhoea, or think you might, and your eyes are red, itchy or painful, or you have pus that sticks to your eyelashes.

Herpes

The Herpes simplex virus can infect the eye too. But it’s usually the cold sore virus, which most people catch in childhood, and it mainly affects the cornea. Read more in “Can you get Herpes in your eye? Symptoms, causes and treatment”

How rare are these? The JAMA review estimates that Chlamydia causes between 1.8% and 5.6% of acute conjunctivitis. Gonorrhoea is rarer still: an eye hospital in Dublin found 14 cases between 2011 and 2013, which was fewer than 1 in every 5,000 people coming to its emergency department. Read more in “Which STIs cause conjunctivitis?”

A baby less than 30 days old with red, sticky eyes needs urgent medical help. Conjunctivitis in the first month of life can come from an infection picked up during birth, including Chlamydia and Gonorrhoea. The College of Optometrists says Gonorrhoea used to be the usual cause, but Chlamydia is now more common.

Usually both eyes are affected, with redness, swelling and watering or discharge. Symptoms caused by Gonorrhoea usually start 2 to 5 days after birth, and those caused by Chlamydia 5 to 12 days after. The NHS says Gonorrhoea in a baby’s eyes can cause blindness if it isn’t treated with antibiotics, so treatment starts without delay.

Testing for STIs during pregnancy, and treating any that are found, helps prevent this.

An eye problem needs checking by a doctor or an optometrist, and the tests below don’t test the eye itself. But if Chlamydia or Gonorrhoea could be the cause, it’s worth testing for a genital infection too, as many people have one without symptoms. Better2Know offers:

• Chlamydia and Gonorrhoea (Urine): tests one urine sample for both infections, from 14 days after sex, with results in 2 days. You can book it at one of our clinics or have a nurse visit you, and there’s also a home test kit.
• Peace of Mind: tests for Chlamydia, Gonorrhoea and Syphilis from 14 days after sex, with results in 2 days. It’s also available as a home test kit.
• Full Screen: tests for 7 infections, including HIV, Syphilis, Hepatitis B, Chlamydia and Gonorrhoea, from 28 days after sex, with results in 3 days (5 days for the home test).

If you test before 45 days, a negative HIV result needs a repeat test. A Syphilis test in the first 3 months after sex can miss the infection, so you may need a repeat test. Not sure which test you need? Our Patient Care Team is available 24/7 on 020 7099 0955.

Is pink eye the same as conjunctivitis?

Yes. The NHS says conjunctivitis is also known as red or pink eye.

Do you need antibiotics for conjunctivitis?

Usually not. They don’t work on viral or allergic conjunctivitis, and most bacterial conjunctivitis clears up without them. Conjunctivitis caused by Chlamydia or Gonorrhoea is different and does need antibiotics.

Can you wear contact lenses with conjunctivitis?

The NHS says not to wear them until your eyes are better. If you wear contact lenses and get a red eye, get urgent medical advice, because it could be an infection of the cornea.

Can you get conjunctivitis from semen in your eye?

It can happen if your partner has Chlamydia or Gonorrhoea. The NHS says both infections can reach the eye in infected semen or vaginal fluid, including on your hands.

Viral and bacterial conjunctivitis look alike, are both contagious and usually get better without treatment within a couple of weeks. Telling them apart matters less than spotting the cases that need help: pain, sensitivity to light, a change to your sight, a red eye when you wear contact lenses, a newborn baby with sticky eyes, or a red eye that won’t clear up. If an STI could be the cause, see a doctor about your eye and get tested.

Book a Chlamydia and Gonorrhoea test at a clinic near you or order a home test kit. Our Patient Care Team is available 24/7 on 020 7099 0955.

Not in the UK? Choose your country from the selector at the top of the page for local tests and prices.

This article originally appeared on 16 June 2023 and was last updated on 5 October 2026.

This article is general information and not a substitute for personal medical advice.

Sources

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