Acanthamoeba keratitis (AK) is a rare but serious eye infection that can affect a person of any age or sex. The incidence of AK is estimated to be approximately 1:250 000. The Acanthamoeba organism exists as 2 forms in the cornea. The first form is the free-living protozoa called a trophozoite; the second is a double-walled cyst that forms in times of stress. Risk factors for AK include contact lens wear; homemade saline solution; water exposure, such as pools, hot tubs, rivers, and saunas; and exposure to vegetative matter or soil. Contact lens wear (especially soft lens wear) has long been known to be a risk factor for AK.

Patients with AK will present complaining of ocular pain, light sensitivity, red eye, blepharospasm, excessive tearing, and possibly reduced vision. Often times, especially in early disease, patients' symptoms will be more significant than what is seen on physical exam. The findings in AK can look very similar to herpetic keratitis, and it is not uncommon for patients to be first treated unsuccessfully with antiviral and/or steroid medications before correct diagnosis of AK is made.