I call this the paranoia effect. F'uck staying inside for that long after baby is born. I had a baby, I'm not an inmate. With both my girls, I was out and about the day after I got home from the hospital. Not long excursions, mind you, as I was still pretty sore from delivering, but after spending three days in a hospital room the LAST thing I wanted to do was stay indoors some more. Talk about cabin fever! Combined with hormones and lack of sleep, a gal can get pretty grumpy being cooped up all day.
ETA: FWIW, my girls didn't get sick, and to this day they rarely do. Their immune systems need to develop too, and the only way to do that is to get them out. Now, am I going to let some obviously sick person near my newborn? Um, no. But it's not going to hurt your baby to take them out of the house.