Help with my persian cat's eye?

I noticed last night that my 6 month old persian kept squinting his one eye a little. When I got up this morning it had some crust on it and the eye is a little red. I know they typically have drainage from their eyes but this was more on the one eye. I cleaned it with a washcloth and warm water and thats when I saw that it seemed irritated and a little red. I am taking him to the vet tomorrow morning but was wondering what it could be? He and my other persian are strictly indoors, both fixed, not de-clawed so they may have been playing and Cozmo got the brunt of it. They get along great. So, I am figuring it may be an infection. How the heck could he get an eye infection? There is no injury whatsoever. Just kinda like when we get pink eye or something. What could it be and should I just continue to clean it till tomorrow's vet appointment? Thank's.