Bacteria and viruses, often just called 'microbes' ('micro' meaning very tiny) are incredibly small and incredibly common. To see bacteria, you'd have to use a powerful microscope that magnifies about 10,000 times. Imagine a bean enlarged that amount. How big would it be? About the size of a large building over 300 feet (100 metres) high. And viruses are much smaller than bacteria. Just one teaspoon of water from a river or the sea normally would contain 50 million! Bacteria used to be thought to be primitive plants. Now scientists know that they are a completely separate group of living things (eubacteria and archaebacteria) which can live in the most extraordinary places. Some can even live in boiling water; others live inside us animals and help us digest food. They were probably the first life on Earth.

Both of these microbes are important in all kinds of ways. One of these, as I bet you know, is that they can cause diseases in plants and animals. For example, one type of bacteria causes tuberculosis (TB). And a type of virus called HIV causes AIDS. Both these diseases are killing millions of people, mostly in poor countries.

Bits of virus called 'promoter sequences' are used in genetic engineering. These are like a switch which turns a gene on. The commonest one is from a virus which infects cauliflowers: cauliflower mosaic virus. Bits of bacteria called plasmids are also used because these tiny loops of genetic material can readily move from one bacterium to another. This is called horizonatal gene transfer and enables bacteria to swap useful genes. Plasmids carry genes which make bacteria resistant to antibiotics or make them manufacture toxins (poisons). They can also carry genes which scientists have inserted -- spliced -- into them. Plasmids allow bacteria to quickly become resistant to antibiotics like penicillin.