Apparently, some people believe it is better to expose their children to full-strength deadly diseases and hope for the best than to vaccinate them. Thankfully, Ars Technica has the explanation of why this is a bad idea:
Though most children who get the itchy, highly contagious viral disease go on to recover after a week or so of misery, chickenpox can cause severe complications and even death in some. Complications include nasty skin infections, pneumonia, brain inflammation, hemorrhaging, blood stream infections, and dehydration.