Thousands of cases of the hospital bug Clostridium difficile have not been reported in recent years, data shows.
C difficile rates have been on the rise
Hospitals have been required to submit C. difficile rates since 2004 - but only for over 65s until April this year when all patients have been included.
Data from 171 NHS trusts in England - over 90% of the total - showed there has been an extra 26,000 cases on top of the 176,450 infections reported.
Tory MP Grant Shapps, who compiled the figures, said they were "shocking".
C. difficile is a bacterium found in the gut of up to 3% of healthy adults and 66% of infants, although it rarely causes problems
Certain antibiotics can disturb the normal balance, allowing the bug to thrive and causing severe diarrhoea and in some cases severe inflammation of the bowel which can be life threatening
He asked all hospital trusts in England for their C. difficile statistics - most have the data for all patients but only publish the over 65s under the government's surveillance scheme.
He said: "This investigation reveals that the number of C. difficile cases has been dramatically underestimated with the government simply ignoring anyone who contracts the infection, but happens to be under the age of 65."
And he added; "It is an absolute scandal that I had to go to these lengths to get the true figures revealed."
C difficile is linked to twice as many deaths as MRSA, and, unlike the so-called superbug, C difficile infection rates are still on the rise.
'No room for complacency'
The number of cases in 2006 rose by 8% on the previous year to 55,000 infections.
A spokesman for the Department of Health said: "There is no room for complacency.
"We are clear - one avoidable infection is one too many and we are determined to ensure that the NHS has in place good hygiene and clinical procedures
"We have recognised that C. difficile is a growing problem - which is why, along with our action programme to tackle all infections, we have a number of specific interventions.
"We have improved the surveillance system to provide better local data, and local trusts have agreed local targets to significantly reduce C. difficile infections."