When I was four, my two year old sister and I had chicken pox. Memorable because my mom was in the hospital having our little sister at the time.
When I was ten, there were six of us and we all got the chicken pox, including the two of us who had presumably already had it.
Apparently, sometimes it just happens. We didn't catch it from illegal aliens, it was just going around.
One of my daughters had it as an infant and then later when she was 6. The doctor said that sometimes when the 1st infection is so mild, you can get it again.
TI have five kids. Three have had Chicken Pox twice.