My friends, and those people I know, have suffered very badly from contracting preventable diseases. Through no fault of their own. To say that they were seriously immune deficient, (when you have no idea what their immune status is or was), is flippant, and dismissive of the real pain that these people have suffered. In particular my friend who lost her 6 week old baby to whooping cough.
A person's immune system, can only deal with pathogens it knows. A disease which is unknown, and that the body cannot respond to quick enough, can cause damage and death.
Perhaps people who do not eat a healthy diet, or the have unsanitary living conditions, (through their own fault or not), may be more likely to contract a disease, but this is not always the case.
A healthy person is also able to contract and be harmed by a disease. A strong immune response can even lead to a worse outcome, particularly in cases of influenza.
I would encourage everyone who can, to improve their immune system, as in most cases this is a good thing. A strong immune system will not mean you will not get any disease, and it certainly wont protect you from an adverse outcome. Not leaving yourself open to getting the disease in the first place is the best way of avoiding an adverse outcome.