Those with good immune systems get sicker easier. The whole point of poisoning people with vaccines is to lower their immune response.
Lets take Tetanus, first for there to be an issue one must have iron in rust form. If so there is a chance that the bacteria can create enough toxins to hide itself from any immune response at least locally to start with.
Once that reaches the blood stream the immune response can't be stopped with local toxins and your own immune system causes the symptomatology of the disease we know as Tetanus.
Its a battle that can't be won with rust in your body..