Viruses have an overwhelmingly negative reputation as a cause of infectious disease, and for good reason.

With a name that translates as “poison” in Latin, they are responsible for some of the most notorious epidemics and health scares in history — including rabies, Aids, foot-and-mouth disease and SARS — as well as the seasonal common cold.

The threat of a flu pandemic is predicated upon the possibility that a simple, chance mutation in an influenza virus could result in a new strain with the potential to kill thousands, if not millions of people.

Often no more than a few molecules of genetic material with a protein coating — and many times smaller than your average bacteria — viruses are found wherever there is life, and rely on host cells to multiply and grow.

But there is increasing evidence that the destructive power of viruses could be harnessed for the benefit of health, and in some cases to prolong lives, rather than end them.

In the past decade there has been an explosion of interest in using viruses as treatments for cancer. This has been as a result of increased understanding on the molecular basis of cancer, and how some of the cell processes associated with disease can be targeted by naturally occurring or genetically manipulated viruses.

After early experiments in mice, certain viral therapies are now being tested in humans, both as single agents and in combination with standard chemotherapy and radiotherapy.

The origin of the most advanced treatment, reovirus, is typically harmless in humans as it cannot attach to healthy cells. But it is deadly to some tumours precisely because it can latch on to a particular genetic switch, known as the Ras pathway, that many cancers use as a means to spread within the body.

Other viruses being investigated include a cause of the common cold and a type of herpes virus that results in cold sores (which, as The Times reported last week, may also play a role in the development of Alzheimer’s disease).

Outside of these trials, it will be some time before any treatments reach the clinic, however.

The main safety concern is that any virus enlisted to the services of doctors may eventually spread out of control and cause side-effects or harm.

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