They probably had it and never knew. Me and a bunch of friends got it and one guy in the same room with all of us didn’t get anything.
My wife has survived our entire family having the rona-me, daughter, two sons, son-in-law, and grandson and last month tested for covid antibodies and it came back negative. She’s been around us, slept in the bed with me when I had the rona, used the same bathroom. The only thing she does is wash her hands often.