And how many kids do you know that get sick with measles, mumps, ruebella, chicken pox, diphtheria, tetanus, pertussis, polio, hep B, and HiB all at once? Yet that's the kind of load being put on an infants immune system. No way that simulates what happens in nature. Maybe the vaccines would be more effective if given individually and the kids system gets to work on one at a time. Of course, it would be a major inconvenience, so it's not likely to happen.