There has been a 22% increase in cases of Clostridium difficile, according to latest figures from the Health Protection Agency.
There were 15,592 cases in patients aged 65 years and over in England reported in the first quarter of 2007, compared with 12,797cases reported in October to December 2006.
The agency said that the higher numbers of vulnerable people admitted to hospital during the winter period may explain the rise.
Latest figures for MRSA show there was a 6.4% fall in the number of cases from January to March 2007. There were 1,444 cases reported compared to 1,542 in the last quarter.
RCN general secretary Peter Carter said: 'Now is not the time to become complacent and take the foot off the accelerator. Clostridium difficile continues to present an increasing and deadly threat to some of our most vulnerable and 'at risk' patients.'