Antibiotics, such as penicillin, are medicines that help to cure bacterial disease by killing infective bacteria inside the body. Specific bacteria must be treated with specific antibiotics, so a doctor has to choose the right one for the infection. Antibiotics have greatly reduced deaths from infectious bacterial diseases. However, strains of bacteria that are resistant to antibiotics have emerged, and this is of great concern.
Antibiotics cannot kill viral pathogens. A cold or flu is caused by a virus, so an antibiotic will not cure it. It is difficult to develop drugs that kill viruses without also damaging the body's tissues, because viruses live and reproduce inside body cells.
Painkillers are medicines that treat the symptoms of a disease, such as pain, but they do not kill pathogens. The person feels better, but the pathogen is still there until the immune system or another treatment removes it.