Why do antibiotics not work against viruses?

Antibiotics are designed to interfere with bacterial biology, not viral biology. Many antibiotics attack bacterial cell-wall synthesis or bacterial ribosomes, which humans and viruses do not share in the same way. Viruses also replicate inside human cells, so treatment usually relies on antivirals, vaccines, or supportive care rather than antibiotics. The discovery of penicillin by Alexander Fleming in 1928 transformed bacterial medicine, but it also made the limits of antibiotics clear. Using antibiotics for viral infections can promote antibiotic resistance without helping the illness.