Antibiotics do not kill viruses, they only kill bacteria.
Many common infections of the nose, throat, sinuses, ears, gastro-intestinal system and chest are caused by germs called viruses. If you are normally well, your body’s immune system will fight off many types of viral infection. Antibiotics are not needed if a virus is causing an infection.
Antibiotics may also cause side-effects such as diarrhoea, rashes or nausea, so it’s best to only take them when necessary.
The overuse of antibiotics in recent years means they’re becoming less effective and has led to the emergence of “superbugs”. These are strains of bacteria that have developed resistance to many different types of antibiotics. Examples include MRSA (methicillin-resistant Staphylococcus aureus) and Clostridium difficile (C. diff).
These types of infections can be serious and challenging to treat, and are becoming an increasing cause of disability and death across the world.
The biggest worry is that new strains of bacteria may emerge that cannot be treated by any existing antibiotics. The more antibiotics are used to treat trivial conditions, the more likely they are to become ineffective for treating more serious conditions
Please see the following information from Public Health England, particularly looking at the usual duration of each of the common viral infections in the second column: