The Marshall Protocol is the brainchild of Ph.D. electrical engineer Trevor Marshall. It claims to be a therapy for chronic inflammatory disease.
It bears no relation to the Marshall Plan (the economic reconstruction plan enacted in Western Europe after World War 2).
 Vitamin D, is the jury still out?
One of the Marshall Protocol's claims is that taking Vitamin D, even if you have a Vitamin D insufficiency, can actually harm your immune system.
There is some clinical and chemical evidence which suggests Vitamin D deficiency may be the consequence, rather than the cause, of several diseases. The Marshall Protocol takes this ball and runs with it, claiming that bacteria and Vitamin D gang up on your body's Vitamin D nuclear receptors (VDR) to shut your immune response down. So, instead of taking Vitamin D if you have both an inflammatory condition and a vitamin D deficiency, the Marshall Protocol recommends taking long-term doses of antibiotics (like minocycline) and olmesartan (a medication for high blood pressure).
These claims are based on computer models of molecular activity, rather than in vitro or in vivo studies. It's one thing to use molecular modeling to open up promising avenues of research; it's quite another to use it as the basis for making actual clinical recommendations. The suggestion to take antibiotics when no positive tests for bacterial infection have been made is particularly disturbing, as overuse of antibiotics can breed antibiotic resistant bacteria.