The number of elderly hospital patients infected by the potentially deadly bug Clostridium difficile jumped by 17.2% in England in 2005.
C. difficile can prove fatal
Figures from the Health Protection Agency show 51,690 cases were recorded among patients aged 65 and over.
The HPA said the increase was partly explained by better monitoring by NHS trusts - but warned the bug did also appear to be more common.
It said the NHS had more work to do to tackle the problem.
C. difficile is a common hospital-acquired infection which usually causes diarrhoea but can lead to fevers or more serious infections.
Older people are particularly at risk, but the figures show that approximately around 13,000 cases were also recorded among younger patients.
Almost 30% of samples in 2005 were caused by C. difficile ribotype 027, which has previously been associated with major outbreaks.
The HPA said there were concerns that 027 was linked to increased severity of symptoms and increased rates among younger people.
The latest figures also show cases of MRSA in England fell by 2.5% to 3,517 in the six months ending March 2006.
Significant decreases were seen in London, the worst affected region, and Yorkshire and the Humber, while there was a continuing slight increase in the North West.
Analysis showed that 25% of patients with MRSA had the disease on admission to hospital.
Dr Georgia Duckworth, from the HPA, said: "Nationally the increase in C. difficile and limited decrease in MRSA cases indicate there is much work to be done, but today's figures show some encouraging signs.
HIGHEST C. DIFFICILE RATES
Kettering General Hospital, Northants: 5.24
George Eliot Hospital NHS Trust, Warwickshire: 4.93
Royal Devon and Exeter NHS Foundation Trust: 4.88
West Herts Hospitals NHS Trust: 4.54
Cases per 100,000 people aged 65 or over in 2005
"Individual trusts, such as the six who reported no MRSA blood poisoning cases at all and others with significant reductions in cases are leading the way in reducing healthcare-associated infection."
Dr Duckworth said it was important to remember that not all hospital-acquired infections were preventable.
"Some of these infections are the price we pay for advances in medicine which allow patients to survive who would have been unlikely to survive their illness a few years ago."
Health Minister Andy Burnham said new powers in the Health Act would be used to deal with organisations that consistently fail to follow good practice guidance designed to cut hospital infections.
He said: "Some trusts have managed to reduce their rates of infection significantly proving that it is possible to get a grip on this problem with focus and good management but elsewhere progress is not good enough and needs to improve.
"Clean, safe care is not an optional extra. We and the Healthcare Commission will not shy away from using the powers in the Health Act to ensure that trusts do everything possible to eradicate avoidable infection."
Andrew Lansley, the Shadow Health Secretary, blamed government targets for rising rates of C. difficile infection by diverting staff focus to other areas.
He said: "The vast increase in C. difficile cases belies the government's complacency in tackling hospital acquired infections.
"It is unacceptable that patient safety has been compromised by government targets and their financial mismanagement of our NHS."
Steve Webb, for the Liberal Democrats, said it was tough to implement effective infection control measures when hospitals were constantly running near full capacity.
"It is damning that the shunting of patients from ward to ward to meet the government's waiting time targets means infections are not isolated at an early stage."
Other figures published by the HPA show that 1,054 orthopaedic patients in England were recorded as picking up an infection in their surgical wound between April 2004 and December 2005 - an overall infection rate of 1.34%.
Infection rates were low for most surgical procedures, for instance 0.7% in knee replacements.
They were highest for hip hemiarthroplasties (3.9%), usually undertaken to repair fractures to the neck of femur.
These patients tend to be older and have underlying illness which affects their susceptibility to infection.