Viruses

Viruses are on the borderline of living and non living things and do not fit into the five kingdoms of the living world. They are much smaller than bacteria and each consists simply of a lump of DNA inside a protein envelope.

There are many different kinds of viruses and they all produce disease in plants and animals by invading living cells and interfering with their normal functions. The viruses can be converted to a crystalline form, in which they can survive for a very long time, but they cannot grow or reproduce on their own. They need the help of the host cells for this. The virus DNA takes control of the cell nucleus and instructs it to make more virus mater. The new virus particles then escape from the host cell and rapidly infect other cells. Human diseases caused by viruses include the common cold, measles, polio and aids