vitamins and particularly C can be toxic. It all depends on dosage.

I read in the New Scientist years ago that what we consider to be toxins do not exhibit a straightline graph of toxicity. Their effect often runs to the negative (ie beneficial) before curving upwards and crossing zero where we would consider the overall effect to be detrimental. Vitamin C and digitalis are obvious examples. So is that well known (medically that is) "depressant" called alcohol. You can die of alcohol poisoning, that is why we might vomit when imbibing heavily.

FWIW it is reckoned: bubonic plague & black death still exist but have evolved to be more parasitic rather than deadly. Because the human survivors also carried a level of immunity, genetically.

E. Coli 0157 has become serious because it was either: around but propagated slowly and was out competed by milder strains of E. Coli which were less devastating or it has evolved to withstand the sanitising agents we use on farms & in shops etc.

The thing about 'flu jabs is that they are vaccines, they prime your immune system. Anti-biotics kill bacteria, mostly. Ever had diarrhea after penicillin? It kills a lot, including the good bacteria.