I bit of mixing going on in this thread as mentioned.
Antibiotics fight bacteria, not viruses.
As for antibiotics in cows, from my understanding (and I'm not an expert here, so feel free to correct me here) they are used in cases when animals are in close quarters to prevent spread of disease, and there is some evidence that they also help cattle grow faster / bigger. Some of the issues with this, as Tracker mentioned, this is giving antibiotics as a prophylactic, meaning before an infection. When done like this, bacteria will mutate faster and gain immunity over the antibiotic which can also affect how resistant the antibiotics are for humans.