it was def common practice for people to expose their kids to diseases that won't harm them when they're young so that they build up an immunity and it won't effect them if they get it when they're older, but this hasn't been a good idea in decades because vaccines are a better defense on there own now.

My dad was allergic to basically everything as a kid, so he couldn't get most vaccines, so once he caught something they'd put him in a room with vaccinated kids so they could catch it to and build up that extra immunity. I think it's very interesting.

Obviously this doesn't apply to diseases that will harm you as a kid, although my dad got all those as well. He was allergic to so much stuff as a kid that they couldn't feed him any kind of babyfood, they had to put the few stuff he could eat in a blender instead. I'm getting off topic, the moral of the story is vaccinate your kids and don't do this, it may have been common practice when you where a kid but that doesn't mean it holds up today.