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considering that Antibiotic kills only a bacteria.

but, why doctors prescribed usually an antibiotic to a person has flu or virus

the reason, nong , is because viral infections are associated with bacterial infection, the latter is an advantageous effect of viral infection., hence the prescription of antibiotics : amoxicillin, penicillin, cephalosporins (1st-4 generation).

to treat  viral infection, you would need antivirals: ribavirin, acyclovir, gangcyclovir.