So the vet finally called me back chewy was positive for giardia

he has been on antibiotic for four days now and hasn't pooped blood since yesterday. The vet said it's contagious. I have two others that seem to be fine, having normal bowels. Should I get them checked. Does anyone have experience. In almost four years my dogs have never been sick and now I'm dealing with this. How serious is it, what should I look for, is there a
shot to prevet this. How do I sanitize the house and backyard.