Yes I always let people know of any sign of sickness so that they can cancel if they wish. If its gastro then it's an automatic cancel. It annoys me no end when people knowingly bring their clearly sick kids over, it is so unfair to the sick kids, my kids and my poor health. Gastro can put me in hospital in a flash so I would be especially livid if someone came over having that. Wouldn't end a friendship over it though.