I just went through this with two of mine, there is a virus going around. And yes, it was DISGUSTING! I have never seen such snotty noses.
Went by the vets and mine recommended to let it just run it's course unless they were running a fever or there were other problems. it has taken a few days but mine are doing better now.... it was odd because two of the four mares got it- one really really bad, but the other two (they all run in the same lot) did not.
KNOCK ON WOOD, so far nobody else on the place has gotten it, so I am HOPING that is going to be it. I have not had the horses anywhere nor have I brought in any strange ones- the vet said it's just going around.
I spent this evening bleaching water buckets, feeders, etc.. to clean them all out to help with the germs hopefully (have been cleaning the water bucket daily like this) and just crossing my fingers that nobody else gets it. It's the season I guess- I just also got over an ugly case of strep myself, after two different antibiotics cause the first ones didnt work... haven't been that sick in a while!!