Finally, three weeks later, a corneal specialist diagnosed Fleming with Acanthamoeba keratitis, a disease that occurs when Acanthamoeba—a common parasite found in tap water, as well as dirt, soil, and any non-sterile body of water, like a pool or lake—infects the cornea, the clear outer covering of the eye. Though she’ll never know for sure how she got it, the most likely cause, Fleming’s doctor told her, was something millions of people do every day: wearing contact lenses in the shower.