MD means Doctor of Medicine, i.e., they completed medical school. Dr. Fauci is a Doctor of Medicine in that he completed medical school and received his MD.

After medical school, doctors will enter a residency program for a specialty, but they are still MDs (DOs are equivalent). Their post-graduate training (residency and fellowship) varies but they are still MDs. MDs who specialize in infectious disease or epidemiology are still MDs. MDs who specialize in family medicine are still MDs.

You are falsely inferring MD means GP (general practitioner) which it may or may not. And you are further falsely inferring that a GP cannot have expertise in virology and immunology, which they likely don't, but they may. If you were to conclude a GP does not have CREDENTIALS to speak authoritatively about virology and immunology, I'd accept that assertion.

Do you have the expertise to evaluate his expertise? Did the person who granted you proof of this expertise also have sufficient expertise to do so? What about the person above that? What is the root of expertise? Plato with his allegory of the cave, casually dismissing claims that come from the wrong mouths in his lofty opinion?

In my mind it would be simpler to evaluate claims as they are rather than bringing the speakers life story into it.