Viruses are organisms that are even smaller than bacteria. Although the size of viruses varies the thickness of an average human hair is approximately 1000 times bigger than a virus. See the SMAC report for more details. Viruses consist of just a string of genetic material (DNA) that is contained by a shell known as the 'viral coat'. They are not cells and they border on being non-living infectious agents as they are only active when in a host cell. Viruses can only replicate (produce more viruses) using the machinery of the cell. This could be a bacterium, plant cell or human cell depending on the type of virus.