1. Viruses might cure cancer

Reovirus, which is relatively harmless to humans, is in trials as a “booster” for anti-cancer drugs. It works by infecting cancer cells that have a mutated form of the Ras gene. The mutated Ras causes runaway cell growth, but also makes the cells vulnerable to the virus. Trials show that reovirus kills these cells, helping anti-cancer drugs do their work.

Researchers in a Nagasaki hospital have found that a virus that occasionally cause leukaemia seems to cut the chances of developing stomach cancer by one-third

Epstein-Barr virus, which is carried by 90 per cent of people worldwide, produces no symptoms in most of us. Sometimes, though it can lead to a leukaemia-like cancer called Burkitt's lymphoma (http://www.cancerhelp.org.uk/help/default.asp?page=119#viruses). But the virus can also be used to battle the cancer. It produces enzymes that can be tagged with radioactive molecules to make the cancer show up in scans. And studies show the virus’s hijacking of cell machinery can be used to burst and kill tumour cells.

2. They can deliver gene therapy

Viruses have been used as carriers to transfer genes into the eyes of four people who suffer from hereditary blindness. The result was significantly improved sight. The hope is that disabled viruses can be used to treat many genetic diseases. Viruses are also being used to deliver genetic material that can help cure AIDS.

3. They can beat antibiotic-resistant bacteria

Doctors have known for around a century that viruses can be used to kill bacteria. In 1917, for example, Canadian microbiologist Félix d'Hérelle cured children of dysentery overnight by giving them a solution containing bacteria-destroying viruses (). In the age of antibiotics, however, viruses were left on the sidelines. With the advent of antibiotic resistance, the viruses are coming off the bench.Bacterium-eating viruses have been harnessed by researchers at University College London to deal with particularly problematic ear infections, for example . It is not yet clear whether bacteria can evolve resistance to attack by virus.

4. They are the driver behind evolution

The way viruses cut and paste DNA has led them to create the diversity essential to evolution by natural selection (http://cvr.bio.uci.edu/learn.html). Many viruses do not just replicate at will and destroy their hosts, but insert themselves into the host’s genome and replicate only when the cell divides. Genetic analysis of every living organism has revealed the presence of viral DNA, and occasionally this is useful information that encodes something the cell can use. In this scenario, viral DNA can radically alter what an organism does – leading to new evolutionary directions.

5. They haven't always been parasites

As scientists discover more and more viruses, they are analysing the DNA inside their protein crystal heads and finding that viruses may once have had an independent existence The trunk of the tree of life is supposed to be split into three branches. The eukaryotes are the advanced organisms whose large and complex cells contained a nucleus that held inheritable information. The bacteria have cells without a nucleus. The archaeans are a bit like bacteria, but have a distinct genetic heritage, and are often found in extreme environments, such as hot springs. Genetic evaluations of viruses suggest they evolved before this split occurred, in which case the parasitism may be a later development.

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