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. Infection by human adenovirus accounts for 65% to 90% of cases of viral conjunctivitis. Viral
Adenoviruses are the most common cause of viral conjunctivitis (adenoviral keratoconjunctivitis). Herpetic keratoconjunctivitis, caused by herpes simplex viruses, can be serious and requires treatment with aciclovir. Acute hemorrhagic conjunctivitis is a highly contagious disease caused by one of two enteroviruses, enterovirus 70 and coxsackievirus A24. These were first identified in an outbreak in Ghana in 1969, and have spread worldwide since then, causing several epidemics. Bacterial
The most common causes of acute bacterial conjunctivitis are Staphylococcus aureus, Streptococcus pneumoniae, and Haemophilus influenzae. Though very rare, hyperacute cases are usually caused by Neisseria gonorrhoeae or Neisseria meningitidis. 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. Allergic
Conjunctivitis may also be caused by allergens such as pollen, perfumes, cosmetics, smoke, dust mites, Balsam of Peru, or eye drops. The most frequent cause of conjunctivitis is allergic conjunctivitis and it affects 15% to 40% of the population. Allergic conjunctivitis accounts for 15% of eye related primary care consultations; most including seasonal exposures in the spring and summer or perpetual conditions. Other
Computer vision syndrome
Dry eye syndrome
Reactive arthritis: Conjunctivitis is part of the triad of reactive arthritis, which is thought to be caused by autoimmune cross-reactivity following certain bacterial infections.
Chemical
Chemical eye injury may result when an acidic or alkaline substance gets in the eye. Alkali burns are typically worse than acidic burns. Mild burns produce conjunctivitis, while more severe burns may cause the cornea to turn white. Litmus paper may be used to test for chemical causes. When a chemical cause has been confirmed, the eye or eyes should be flushed until the pH is in the range 6–8. Anaesthetic eye drops can be used to decrease the pain.Irritant or toxic conjunctivitis is primarily marked by redness. If due to a chemical splash, it is often present in only the lower conjunctival sac. 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. A slit lamp examination is likely to show evidence of anterior uveitis. Other
Inclusion conjunctivitis of the newborn  is a conjunctivitis that may be caused by the bacterium Chlamydia trachomatis, and may lead to acute, purulent conjunctivitis. However, it is usually self-healing. Causes
Infective conjunctivitis is most commonly caused by a virus. Bacterial infections, allergies, other irritants, and dryness are also common causes. Both bacterial and viral infections are contagious, passing from person to person or spread through contaminated objects or water. Contact with contaminated fingers is a common cause of conjunctivitis.