Viruses are much smaller than other living things like bacteria, so small that it would take around one hundred viruses laid end to end just to make the length of a bacterium! Viruses are not really alive. They do not do all of the things that living things do. They can only make more copies of themselves when they are inside living cells.
Viruses often kill cells, and also make you ill. Lots of diseases are caused by viruses, the most famous ones being the flu and many colds.