[1][3] If visual loss, significant pain, sensitivity to light or signs of herpes occur, or if symptoms do not improve after a week, further diagnosis and treatment may be required.

[3][7] Viral conjunctivitis is often associated with an infection of the upper respiratory tract, a common cold, or a sore throat.

Other associated signs may include pre-auricular lymph node swelling and contact with another person with a red eye.

Symptoms result from the release of histamine and other active substances by mast cells, and consist of redness (mainly due to vasodilation of the peripheral small blood vessels), swelling of the conjunctiva, itching, and increased production of tears.

[7] Common bacteria responsible for bacterial conjunctivitis are Staphylococcus including Staph aureus, Streptococcus such as strep pneumoniae,[11] Haemophilus species and Moraxella catarrhalis.

[13] Bacterial conjunctivitis usually causes a rapid onset of conjunctival redness, swelling of the eyelid, and a sticky discharge.

The gritty or scratchy feeling is sometimes localized enough that patients may insist that they have a foreign body in the eye.

Cases of bacterial conjunctivitis that involve the production of membranes or pseudomembranes are associated with Neisseria gonorrhoeae, β-hemolytic streptococci, and Corynebacterium diphtheriae.

With some chemicals, above all with caustic alkalis such as sodium hydroxide, necrosis of the conjunctiva marked by a deceptively white eye due to vascular closure may occur, followed by sloughing off of the dead epithelium.

Bacteria may also reach the conjunctiva from the edges of the eyelids and the surrounding skin, from the nasopharynx, from infected eye drops or contact lenses, from the genitals or the bloodstream.

[22] Herpetic keratoconjunctivitis, caused by herpes simplex viruses, can be serious and requires treatment with aciclovir.

[23] The most common causes of acute bacterial conjunctivitis are Staphylococcus aureus, Streptococcus pneumoniae, and Haemophilus influenzae.

Chronic cases of bacterial conjunctivitis are those lasting longer than 3 weeks, and are typically caused by S. aureus, Moraxella lacunata, or Gram-negative enteric flora.

[citation needed] Conjunctivitis may also be caused by allergens such as pollen, perfumes, cosmetics, smoke,[25][unreliable medical source?]

[28] Allergic conjunctivitis accounts for 15% of eye related primary care consultations; most including seasonal exposures in the spring and summer or perpetual conditions.

[33] Although conjunctival scrapes for cytology can be useful in detecting chlamydial and fungal infections, allergies, and dysplasia, they are rarely done because of the cost and the general dearth of laboratory staff experienced in handling ocular specimens.

Conjunctival incisional biopsy is occasionally done when granulomatous diseases (e.g., sarcoidosis)[34] or dysplasia are suspected.

[citation needed] Blepharoconjunctivitis is the dual combination of conjunctivitis with blepharitis (inflammation of the eyelids).

It is clinically defined by changes of the lid margin, meibomian gland dysfunction, redness of the eye, conjunctival chemosis and inflammation of the cornea.

[36] Some more serious conditions can present with a red eye, such as infectious keratitis, angle-closure glaucoma, or iritis.

However, if any of these symptoms is prominent, considering other diseases such as glaucoma, uveitis, keratitis, and even meningitis or carotico-cavernous fistula is important.

Vaccination against some of the causative pathogens such as Haemophilus influenzae, pneumococcus, and Neisseria meningitidis is also effective.

[41] For allergic conjunctivitis, cool water poured over the face with the head inclined downward constricts capillaries, and artificial tears sometimes relieve discomfort in mild cases.

[44] Antibiotics are also recommended for those who wear contact lenses, are immunocompromised, have disease which is thought to be due to chlamydia or gonorrhea, have a fair bit of pain, or have copious discharge.

[citation needed] When investigated as a treatment, povidone-iodine ophthalmic solution has also been observed to have some effectiveness against bacterial and chlamydial conjunctivitis, with a possible role suggested in locations where topical antibiotics are unavailable or costly.

[45] Conjunctivitis due to chemicals is treated via irrigation with Ringer's lactate or saline solution.

Chemical injuries, particularly alkali burns, are medical emergencies, as they can lead to severe scarring and intraocular damage.

[47] Rates of disease is related to the underlying cause which varies by the age as well as the time of year.

This event marked one of the largest outbreaks of Pink Eye in the country's recent history.