In season 1, Stephanie gets chicken pox & Joey states he can't get them because he's immune but ends up with them anyway. However, in season 4, when Joey performs in Vegas, his dad tells him they watched cartoons for 2 weeks when Joey had chicken pox when he was 3. How could he have gotten them as an adult if he had them at 3?
it is possible to get chicken pox when you are older even though you had them as a kid. Having them at a young age can possibly lead to having them in the future, or possibly having them mis-diagnosed as a kid, and then anything is possible.