Why do you think a lot of doctors do not give the shots when the individual is currently sick with something else? It’s because the immune system is applying an active effort to something else and that is where the risk of overload actually is, when you are actually sick and struggling with another disease. MMR is three related germs, so it doesn’t overload to put closely genetically related viruses in the same shot, if the individual is healthy and not struggling with something else at the moment.
yes, and yes their are combinations that have been around for decades and combinations that haven’t yet a child may get three shots each with three viruses. That’s a lot.