In utero the human immune system produces a vast assortment of antibodies via random shuffling of DNA. All those antibodies that “report back” that they have found a binding partner are destroyed, because they recognize “self”.
Thus a child is born with millions of different antibodies, none of which recognize “self”.
A vaccine is designed to “find” the appropriate antibody developed through random shuffling of DNA that will bind to the 3-D structure of the pathogen, and stimulate the body to produce a TON of that particular antibody (against measles for example, such that if the individual later gets infected by actual measles, the antibody will be present in enough quantity that you will not even get sick).
All this ‘nutrition to strengthen the immune system not toxins’ is silly. The immune system is “strengthened” by getting appropriate ‘information’ via vaccinations of what antibodies to proliferate to avoid measles, chicken pox, small pox, etc, etc.
When the immune system encounters a pathogen, it will either fight it off and the organism will recover, or it will succumb and the organism will die. The health of the immune system is extremely important in the immune response. A healthy person who has given its body the fuel (nutrition) it needs will manifest a stronger immune response than someone who is immunocompromised. While I agree with your point, the stronger immune system will still have a more favorable immune respose. And as I said in a previous post, these pathogens will all be encountered at one time or another. Some will get the disease, others not. Genetics certainly plays a role, but so does the overall health of the organism.