The Conjunctivitis Curse is a curse that causes irritation in the target's eyes, causing them to swell shut like the infection conjunctivitis, commonly known as "pink eye."
Viktor Krum used this curse on a dragon during the First Task of the Triwizard Tournament. However, because she could not see, the dragon crushed some of the eggs she was protecting, and Krum lost points. Sirius Black, while talking to Harry through the Gryffindor Common Room fireplace, was going to suggest that Harry Potter use this curse on his dragon as well, but was interrupted by Ron Weasley coming down the stairs because he heard voices.
Olympe Maxime used this to force Golgomath's companions to drop Rubeus Hagrid when the giants became hostile towards them.