Staphylococcus aureus - or staph for short - is a bacterium. MRSA is a staph bacterium that has become resistant to the antibiotic methicillin. MRSA is usually not harmful. An estimated 30 to 40 per cent of people have been 'colonised' by MRSA. They carry it on their skin or in their nose, and never develop any symptoms.

It occurs most commonly in hospitals, but is also starting to emerge elsewhere. Overseas, it's been found in homes for the elderly, prisons and other crowded environments. It can spread quickly from person to person by direct and indirect contact. Those with weakened immunity are the most at risk, including patients in intensive care, elderly patients, children and those who've already been colonised by MRSA, but haven't shown signs of infection.

It becomes harmful when it enters the bloodstream via a cut or wound or when the skin is otherwise broken, for example by a drip or catheter. Once in the bloodstream, it can lead to a range of problems, including septic wounds, bone infections, heart valve and bloodstream infections, and toxic shock syndrome. These infections are, at best, difficult to treat and, at worst, fatal.