Very infectious between sheep, particularly when trough feeding etc.
Best thing to keep those isolated.
Clears up on its own in a couple of weeks.
If lots of the flock get it then just ensure that they're in a safe environment (eg, no ditches to fall in) and carry feed to any that are too blind to get to trough.
Pinkeye itself won't do any harm, but if pregnant ewes don't feed because of it then can lead to probs.
It's very common at this time of year, and at housing.