Viruses

Microorganisms smaller than a single-celled bacterium, viruses cannot grow or reproduce apart from a living cell. A virus invades living cells and uses their chemical machinery to keep itself alive and to replicate itself.

 

Viruses may reproduce with fidelity or with errors (mutations). This ability to mutate is responsible for the ability of some viruses to change slightly in each infected person, making treatment more difficult.

 

Reference: MedicineNet.com.