I got a corneal ulcer once. I have no idea what cause it but it scared me witless. I was at college and tried to see a doctor at the college surgery but , being London, I lived in the wrong postcode so they refused to see me. I had to track down and register with a non-college surgery then be forwarded to a specialist eye clinic.
They gave me some drops which helped cure it after a few days.
I wouldn't even try to wear contacts now and Im afraid that laser eye surgery would not heal up well so Im stuck with prescription glasses.