A genetic achilles heel that helps viruses invade the body and trigger cancer has been uncovered by scientists.

Some viruses learned to protect themselves by exploiting a natural "brake" on the immune system, research has shown. The process is complex and involves microRNAs – small chunks of genetic material that help regulate gene activity.

One of their roles is to dampen immune responses so they do not run out of control, leading to harmful inflammation.

The UK study found that some viruses can boost microRNA activity to suppress the immune system's reaction to them. One herpes virus with this ability can trigger a cancer called Kaposi's sarcoma in susceptible individuals. Another is the herpes virus which causes cold sores.

Scientists are investigating the role of microRNAs which, like the RNA molecules involved in protein synthesis, are derived from DNA. But instead of performing the usual RNA function of carrying protein-making instructions from the genetic code, they suppress the action of genes.

Scientists estimate a third of genes may be regulated by microRNAs. The new study shows how viruses are able to "upregulate" the production of immune system microRNAs to facilitate infection.

The head of the study, Dr Dimitris Lagos, from the Cancer Research UK viral oncology group at University College London, said: "The viruses we tested have evolved with humans for millions of years and use a variety of biological tricks to establish life-long and mostly harmless infections.

"We discovered that it is likely that other viruses, which can cause diseases including cancer, exploit the tiny molecules present in everyone's DNA, called microRNAs, to turn cells into a viral 'hotel' which they can check into, to cause infection and spread."

The research was reported in the journal Nature Cell Biology.

Dr Lesley Walker, director of cancer information at Cancer Research UK, said: "This is intriguing research which unravels an ancient molecular mystery – and provides a hot new therapeutic target to prevent cancer-causing infection from viruses.

"Infectious viruses and human DNA have been around for so long that a relationship has developed and viruses can cheekily use the bits of our DNA that we often think of as rubbish for their own benefit. Now we know how they are using our DNA we can research ways to put the brakes on it."