Virus

A virus is a non-cellular infectious agent that has two characteristics: It has genetic material (either DNA or RNA) inside a protective protein coat, and it cannot reproduce itself. To reproduce itself, it needs to infect a cell. The genetic material is inserted into the cell, and the reproductive machinery of that cell is then effectively "hijacked": the cell starts to produce components of new virus particles. These components spontaneously self-assemble and the new virus particles can then infect other cells, where the same process happens.

Examples of viruses include the influenza virus, the common cold, smallpox and polio.