Is the pupil of the affected eye dilated? There is a condition which is similar to herpes, called ocular herpes, which affects the eye, dilating the pupil and causing severe sensitivity to light which requires surgery to correct it. Not attending to it can cause scarring of the cornea and ultimately affect the eyesight. It's usually caught from water, mainly swimming pool water which is not treated hygienically or has signs of stagnation. Only a few people are prone to this condition, the main symptom being a severely dilated pupil and bloodshot/closure of the affected eye.