So I guess the thinking is that things get trapped under the contact lenses. When you blink or pour water over your eyes, you would normally flush nastiness away from your eyes. With the lenses on, it blocks the flushing action, which gives the parasite more time to establish itself.

I suppose this is probably also true for most bacteria or foreign particles too. I imagine it’s also one of the risks of just wearing contacts for too long without changing them.