Except for a handful of antibiotics tailored to fight a very few specific viruses, antibiotics work only against bacterial infections. Bacteria are much easier to kill than viruses, most of which are very simple organisms. Unfortunately, the indiscriminate use of antibiotics has led to the development of antibiotic-resistant strains of bacteria. Some doctors handed out antibiotics with a lavish hand when they were introduced and even later, when evidence that indicated their role in creating resistant bacteria was mounting. Patients often demand antibiotics to treat an illness for which they are ineffective and will shop for a doctor who will write that prescription. Such doctors are, unfortunately, not too difficult to find. Antibiotics must also be used exactly as prescribed. People often stop taking them when they begin to feel better or if they feel slightly ill from the drug. This means that the bacterial infection is not completely killed off and the gradual exposure of the bacteria to low doses of antibiotics only serves to make them immune. (The first doctor that I recall, in the late 50s and early 60s, used antibiotics very sparingly and insisted they be taken as directed. He did not hand them out to patients who came into his office with a cold. It turned out to be very sound advice.)

That would depend entirely on one's definition of an 'antibiotic'. Literally the term means 'anti-life' and could be taken to span anti-viral and anti-fungal treatments as well as anti-bacterial agents. However in common usage (and therefore the most widespread understanding of the term), antibiotics applies only to anti-bacterial agents and these do not combat viruses. This is because anti-bacterial agents target the growth and replication of a bacteria or its components (such as their cell wall). This is effective against bacteria as they can grow and replicate by themselves and have very different structure and chemistry to human cells and can therefore be targeted with little danger to our own bodies. This approach is ineffective against a virus as they are not capable of reproduction outside of a host cell. This means that any attack on a virus and it's replication process involves an attack on our own cells, clearly not an advisable approach.

No because they are specific to the bacteria's cell wall.