Antibiotics work by affecting things that bacterial cells have but human cells don't. For example, human cells do not have cell walls, while many types of bacteria do. The antibiotic penicillin works by keeping a bacterium from building a cell wall. Unfortunately, they are less effective against viruses than they are against bacteria. Antibiotic drugs kill bacteria by disrupting their cell walls. But virus' external covering, known as the viral envelope, is almost identical to the host cell's membranes, making them difficult to target