Breaking a precept doesn't really have anything to do with the other. Doesn't matter whether it is an animal, a human or a bacteria. Preventing the dying of another being is not the point.
It has to do with your intentions. Is your intention to harm another being? Or is your intention to take care and relief yourself of suffering? Death can never be avoided, unnecessary suffering on the other hand, can.
As a side note: virusses are not living and antibiotics don't work on those anyway. Antibiotics only work on bacteria.