Well I would have to disagree, i have had chicken pox and rubella as a child and as i am pg i have just had my blood work done and all the markers say that i am immune - no booster needed, no additional exposure required. Most of these 'diseases' are NEVER forgotten by your bodies immune system, in fact some of them are stored else where in your body.
I actually never said the body forgets all the diseases it has ever had. I only pointed out the main three which is why they reccomend regular ten yearly boosters for them.