Acanthamoeba keratitis - a rare disease in which amoebae invade the cornea of the eye; affects up to 1 in 10,000 contact wearers worldwide.

*the 1 in 10,000 number is very high end estimate. From the study:

The estimated rate of Acanthamoeba keratitis is 1 per 250,000 people in the United States, although rates vary among studies: from 1.65-2.01 per million population up to 1 per 10,000 people who wear contact lenses.