Viruses

A virus is not a cell: it is a little genetic material inside a coat of protein, far smaller than a bacterium. Outside a living cell it does nothing at all - some viruses can even be made into crystals - but once inside a cell it makes the cell build hundreds of new viruses, which burst out to enter other cells. Is it alive, then? It has genes and copies itself like living things, but can do none of life's work on its own, so it sits on the border. Viruses cause colds, flu, measles and AIDS. HIV, the virus behind AIDS, grows in white blood cells and weakens the immune system; it passes through blood and certain other body fluids, for example on a shared razor or needle, and never through a handshake, a hug or sharing a classroom.