Antibiotics are ineffective against viruses because viruses do not have cells. Viruses are infectious agents that live within the cells of other living things. Antibiotics work by breaking down the cell walls of bacteria or interfering with the bacteria's ability to repair its cell's DNA, according to How Stuff Works.
According to the Centers for Disease Control, individuals should not take antibiotics for viral infections because this ineffective treatment cannot cure the infection or make the person feel better. The CDC also warns that taking antibiotics for viral infections may lead to future antibiotic resistance when the individual is afflicted with a bacterial infection.