MRSA is a potentially lethal bacterium that causes infections in humans.
It is difficult to combat because it has developed a resistance to certain antibiotics.
MRSA is resistant to antibiotics
Methicillin-Resistant Staphylococcus Aureus usually strikes in hospitals where sicker and weaker people tend to be in close proximity.
Infections occur where there is opportunity for MRSA to get into the body, such as at surgical wounds or where a catheter or needle is inserted.
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MRSA can exist harmlessly on people's skins without their even knowing it, but if it infects someone who is already ill or weak, it can kill.
It is no more dangerous than other forms of Staphylococcus aureus.
But because it has developed resistance to methicillin and other antibiotics, it is much more difficult to treat.
The National Audit Office estimates hospital-acquired infections contribute to some 5,000 deaths annually.
Of those caused by the Staphylococcus aureus family, most are known to be due to the MRSA strain.
Between 2003 and 2004, mentions of MRSA on death certificates increased from 955 to 1,168.
The government has imposed new hygiene standards on hospitals to try to stem its growth.
Combating the infection
The NHS is targeting ward cleaning and personal hygiene.
Tactics include providing disinfectant hand-rubs and encouraging more vigilance among patients and visitors.
Doctors are now more careful about prescribing antibiotics only when essential, which should help slow the evolution of resistance among bacteria.
Growth of MRSA
Mandatory reporting by doctors of MRSA infections in the bloodstream - where the consequences are often more serious - was not introduced until 2001. Cases rose to a high of 7,684 in 2003/4.
The following year saw a fall to a record-low of 7,212 infections - but that is still well short of the target of fewer than 4,000 cases by 2007-8.
The latest figures show that there were 3,517 infections between October 2005 and March 2006.
More than 40% of Staphylococcus aureus infections in the UK are MRSA - one of the highest levels in Europe.
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The National Audit Office estimates that hospital-acquired infections, including MRSA, are costing the NHS about £1bn a year.