This is a great question. Antibiotics may have an effect on viruses. It all depends. For the most part, antibiotics were created to fight foreign bacteria in the body. Antibiotics work in one of two ways. They can either hinder the bacteria from growing or it can kill the bacteria directly. In either case, the goal is to defend against foreign bacteria that may get invade the body. For instance, if a person falls and his or her wound has dirt and other foreign substances, he or she may have foreign bacteria in his or her body. This person's body will seek to treat it naturally. If this does not work, then antibiotics might be necessary.
When it comes to viruses, modern medicine has a few antibiotics that only kill a handful of viruses. Viruses are much harder to kill for several reasons, but the most important reason is because they mutate so quickly. The rate of mutation can be frighteningly fast.