1 The people who live furthest away will get sick

 The day of the party, you'll get between 3 and 5 "ahhh, sorry I can't make it, I'm sick or my girlfriend's sick or whatever anyway can't make it have fun!!" messages from people who originally said they were coming. Coincidentally, all these people live the furthest away, so you know what's really going on: They're all telling the truth and there's a powerful virus affecting the far-from-you areas in your town. And it also infected your flakiest friend.