why are antibiotics not effective against …

Antibiotics are medicines used to treat infections caused by bacteria. Antibiotics don't work against infections caused by viruses, such as colds and flu. So there's no point taking antibiotics to treat a cold or flu.Common cold is caused by a pathogen called as virus.

Virus is a non-living substance when present in the external environment. Virus becomes active when it enters the host cell. As it has only genetic material and a protein coat, it has to depend on host cell machinery for replication. In this way it overtakes the host cell apparatus. Anti viral drugs should target exactly the virus. But since viral particle lives inside the host and gets integrated into the host cell, application of antiviral therapy through drugs is very tough. Anti viral drugs may also kill the host cell also. This is the reason why it is antibiotics have no effect on common cold.

Antibiotics are not effective to common cold because common cold is caused by the virus and antibiotics are effective on bacterias. It stops the bacteria by dividing. It makes a layer on the cell wall of bacteria but virus don't have cell wall and they use human cell to divide.