A protein-wrapped genetic material, which is the smallest and simplest life-form known. A virus is smaller than bacteria, and 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. It 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. Examples: Norovirus, Hepatitis A.

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