Having a properly functioning immune system helps you to weather infections, but it cannot prevent you from getting sick from a pathogen if you do not have pre-existing immunity against that specific pathogen.
The flu virus recognizes and binds to specific cell surface receptors in your respiratory system. Those receptors carry the virus inside the cell, where it takes over and forces that cell to produce new viruses. The only way your body can prevent this process is by having circulating antibodies that recognize and attach to that virus, which physically prevents the virus from attaching to the cell receptors. Once the antibodies attach to the virus, they signal other components of the immune system to destroy the virus.
No amount of vitamin D or "immune system enhancement" can substitute for the presence of specific antibodies that recognize and neutralize specific viruses.
“Having a properly functioning immune system helps you to weather infections, but it cannot prevent you from getting sick from a pathogen if you do not have pre-existing immunity against that specific pathogen.”
A fairly standard medical view.
If your body has the correct ph level and nutrition, germs will find it tough to gain a foothold and flourish in a bad environment. Vitamin D is part of a good terrain.
If your terrain is strong, your body will process germs and move them out quickly.
In other words, the other side of the coin you did not address is terrain of the body. Terrain is always s the first line of defense as part of your immune system. I refer you to the work of Antoine Béchamp.
Completely untrue. Tell it to those with HIV or undergoing chemo.
It's not "weather" btw. That's about when it is going to snow.
The only way your body can prevent this process is by having circulating antibodies that recognize and attach to that virus, which physically prevents the virus from attaching to the cell receptors.
Quite often people manufacture those antibodies and the primary response is more than adequate. For instance I had measles, mumps and chicken pox as a child and overcame them all without a vaccine.
Vitamin D, in the form of cod liver oil, used to be used to treat TB patients. It has been found to change immune function in many ways which I am not going to elaborate here....but I did find this to support that statement.
Vitamin D deficiencies are real. I once had to take prescription vitamin D - something like 20,000 units every week for a few weeks.
Stop giving medical advice - you could harm people. Back your statements up with links to show your opinions are more than that.