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Eye Diseases

Common Disorders of the Eye

Principal Microbial Causes

Many different types of organisms may cause eye infections, while the types of infections they cause fall into four categories: blepharitis, conjunctivitis, keratitis, and dacryocystitis.
Although there are many microorganisms that are capable of causing eye infections, certain eye conditions have principal microbial causes. For example, blepharitis, or inflammation of the eyelids, can be caused by the arthropod parasites Demodex or Phthirus species; HSV-1, HSV-2, or M. contagiosum viruses; or Staphylococcus or Moraxella bacterial species. Keratitis, or inflammation and infection of the cornea, has many infectious causes, but the most common include the parasite Acanthamoeba; HSV-1 or HSV-2 viruses; Pseudomonas, Staphylococcus, Streptococcus, or Moraxella bacterial species; and Fusarium, Aspergillus, or Candida fungal species.

Primary Causes of External Eye Infections

Type of Infection Bacteria Virus Parasite Fungi
Blepharitis Staphylococcus species
Moraxella species
Herpes simplex 1 or 2
Molluscum contagiosum
Phthirus pubis
Demodex species
Conjunctivitis Chlamydia trachomatis
Staphylococcus aureus
Streptococcus species
Neisseria gonorrhoeae
Haemophilus influenzae
Moraxella species
Herpes simplex 1 or 2
None None
Keratitis Pseudomonas aeruginosa
Staphylococcus aureus
Staphylococcus epidermidis
Moraxella species
Herpes simplex 1 or 2 Acanthamoeba Fusarium species
Aspergillus species
Candida albicans
(lacrimal gland infection)
Staphylococcus species
Streptococcus species
None None None

Various species of bacteria, viruses, parasites, and fungi cause ocular infections, but certain microbes more commonly affect a certain part of the eye than others. For example, fungi rarely cause blepharitis but are implicated in keratitis infections.


Conjunctivitis is an inflammatory condition of the eyelid-lining conjunctiva with numerous infectious and noninfectious causes.

The term conjunctivitis means "inflammation of the conjunctiva." It is commonly called "pink eye" because the eye becomes visibly red in appearance. Often, an assumption is made that the cause of conjunctivitis must be infectious in nature. However, this is not always the case. Conjunctivitis may be allergic in origin, caused by exposure to the same causes of allergic rhinitis in the environment: dust, pet dander, pollen, smoke, etc. Conjunctivitis may be due to a chemical burn of the eye. A splash of bleach, other household cleaner, or sap from the milkweed plant may get into the eye and cause severe inflammation. Conjunctivitis may be due to prolonged exposure of the eye to air. This commonly happens as people age and the lower eyelid loses elasticity and drops away from the eye, preventing complete closure of the eye with each blink. If an eye cannot be fully closed with each blink, the exposed portion of the eye becomes inflamed. Conjunctivitis may occur due to a foreign body in the eye, such as the overwearing of contact lenses.

However, when conjunctivitis is infectious in origin, the most common agents are usually bacteria or viruses. Bacterial conjunctivitis can be spread through direct contact with an infected person or through contact with a contaminated surface. Viral conjunctivitis spreads through airborne transmission or as a consequence of respiratory infection. Bacterial conjunctivitis can be distinguished from viral by the production of eye discharge, which is more abundant and thicker than in the viral form. The particular infectious agents can vary widely depending on the age of the human host.

Bacterial and Viral Conjunctivitis

Infectious conjunctivitis, or pink eye, is a highly contagious infection of the conjunctiva, the eye's membrane lining, caused by both bacteria and viruses. The bacterial form produces a thick, yellowish discharge, and the viral form produces a thin, watery discharge.
In neonatal (defined as within the first month of life) conjunctivitis, Chlamydia trachomatis, Streptococcus viridans, and Staphylococcus aureus are the three top causes of infection. Pregnant women with chlamydia are at risk of giving birth to infants with neonatal conjunctivitis, a congenital disorder. Congenital means present from birth, as in a disease or physical trait. Left untreated, neonatal conjunctivitis can cause blindness.

In adults, bacterial conjunctivitis is less common than viral (adenovirus, rhinovirus) conjunctivitis but can be classified based on the course of onset and severity. For example, mild bacterial conjunctivitis infections that develop over days to weeks are commonly caused by S. aureus, Moraxella lacunata, Pseudomonas, and Proteus species. Causes of acute or subacute conjunctivitis occurring over hours to days and producing moderate to severe inflammation include the organisms Haemophilus influenzae and Streptococcus pneumoniae. Conjunctivitis that develops severe inflammation in under 24 hours can be caused by Neisseria gonorrhoeae or Neisseria meningitidis.

Chlamydia trachomatis is a bacterium that exists as three different serotypes, each causing a different conjunctivitis syndrome. Serotypes A–C cause trachoma; serotypes D–K cause adult and neonatal inclusion conjunctivitis; and serotypes L1, L2, and L3 cause lymphogranuloma venereum. C. trachomatis is an obligate intracellular pathogen, which means it cannot be easily isolated through standard culturing techniques. Direct visualization is possible with a particular type of staining method, called a Giemsa stain. Trachoma occurs in communities with poor hygiene and substandard sanitation. It affects 150 million people worldwide and is the leading cause of preventable blindness. It is endemic in the Middle East and developing regions. It spreads via discharge from the eyes or nose of an infected person, through household fomites (e.g., use of towels or other items), or by flies. A severe inflammatory follicular reaction occurs on the upper lid conjunctiva that leads to scarring. In addition, the corneal epithelial cells become infected, called keratitis, and neovascularization, or blood vessel growth onto the cornea, occurs, all of which lead to more scarring and can affect the vision. The treatment for trachoma is tetracycline ointment applied twice daily for two months and oral azithromycin given as a one-time dose.

Adult chlamydial conjunctivitis is a sexually transmitted disease and is transmitted by direct or indirect contact with infected genital secretions. Conjunctivitis symptoms occur one to two weeks after exposure and involve inflammation of the lower eyelid conjunctiva, along with discharge and swollen lymph nodes. The cornea can also develop signs of keratitis, or inflammation of the corneal cells. This disease can resolve spontaneously over 6–18 months, but different regimens of oral antibiotics are usually given, some of which include azithromycin or doxycycline. People diagnosed with chlamydial conjunctivitis need to be tested for other sexually transmitted diseases, including syphilis and gonorrhea, as well as have their sexual partners get tested.