I think that a healthy immune system is capable of protecting a person without always developing an immunity to something through exposure. For example, Methicillin-Resistant Staphylococcus Aureus is very common. I have worked very closely with patients who have had this disease. You could say that my immune system was exposed and developed immunity to this, but I have never had symptoms of this, which would mean my immune system would have to be healthier than the many people who contract this bacteria because of staying in a hospital, unfortunately.
So the health of my immune system is why I have spent years working around an antibiotic-resistant bacteria and have never had to be treated for this with antibiotics that were not Methicillin. It is also true that everyone who comes to the hospital will not be screened for this. Only the people who are at an increased risk because they have some other factor that serves to weaken their immune system like a prior hospitalization.
It is also true that we minimize the risk of spreading this bacteria by discharging patients from the hospital sooner, wearing protective equipment like gloves, and washing our hands, which I do. Yet, other patients will still contract MRSA because of people like myself who are working with the patients are diagnosed with this. Although, I cannot think of one coworker who has had MRSA themselves without some other factor in play like having another condition that weakens the immune system or having some kind of operation or procedure.
So your health will play a role in whether or not you are able to resist illnesses. Some people are predisposed to be at greater risk because they have some factor that weakens the immune system. The only other route that creates another level of difficulty is the airborne stuff because they are more difficult to quarantine. Tuberculosis is all but eradicated at this point without there being any vaccine for it through quarantine and antibiotics, not through the body developing any type of immunity to this.
The flu isn't even an airborne precaution, it is a droplet precaution, which basically means cover your mouth when you cough and wash your hands rather than use hand sanitizer if you come in contact with mucus. So there are still more ways to isolate this and prevention measures that can be implemented that a full-blown airborne illness doesn't have. People are more at risk of flu if they have a weakened immune system as well. Many people can survive flu because they have a healthy immune system.
On the other hand again, chicken pox is an airborne virus that most everyone born before the vaccine had and survived, so airborne doesn't mean that the immune system isn't capable of overcoming it either. So when Gates is talking pandemic, it is hard to know what he is talking about. In reality, we are already facing several pandemics at the moment. It is a little bit concerning to me, and then again, it is also just another day at the same time or just a matter of adding one more to the list.