Can you get Chicken Pox more than once?

You're not supposed to but a lot of people do get them twice...or 3 times maybe(3 is rare). I've had them twice by age 15. My theory is that the sickness evolves over time; or if you don't get them bad enough the first time, you don't build an as high immunity to them. I've also heard they get worse with age. Which is dangerous because they can get to your brain.