Contrary to popular belief, antibiotics have no effect on a virus. Most antibiotics interfere with the reproduction of bacteria, hindering their creation of new genetic instructions or new cell walls. Because viruses do not carry out their own biochemical reactions, antibiotics do not affect them.
Immunizations work by pre-infecting the body so it knows how to produce the right antibodies as soon as the virus starts reproducing. Also, because viruses reproduce so quickly and so often, they can often change slightly. Sometimes, mistakes creep into their genetic instructions. These changes might alter the protein coat slightly, so one year's batch of vaccine might not be as effective against the same type of virus next year. This is why new vaccines must be produced constantly to fight viral infections and prevent outbreaks.
You may have heard of outbreaks of Ebola virus or West Nile virus that have left many people dead. Influenza has killed many people in the past (early in the 20th century), and debate rages over when the next major flu epidemic will occur in the United States. Not all viruses are deadly. For example, people get colds all of the time and do not die. However, even these seemingly harmless viruses can be deadly to a person who already has a weakened immune system -- people with AIDS, cancer patients taking chemotherapy, elderly people or newborns. We have to take care not to spread viruses to these especially susceptible people.