Cochrane: Acanthamoeba are microscopic, free-living, single-celled organisms. Infection of the eye by these organisms is known as Acanthamoeba
keratitis (AK). Although very rare, if left untreated AK can lead to
the loss of vision in the eye or, in extreme cases, loss of the eye
itself. Contact lens wear is the most common risk
factor for AK. The most common symptom of AK is severe pain, which is
out of proportion to the clinical signs and associated with sensitivity to light, blurry vision, and tearing.