A quarter of cases of MRSA bloodstream infections in hospital patients occur in patients who have just been admitted, a study shows.
MRSA appeared to cause infection weeks after patients left hospital
But the Oxford team say patients were not infected by the strain of the superbug which exists in the community.
Instead they say the bacteria had probably lain "dormant" in the patients since a hospital stay some time ago.
In the British Medical Journal they say doctors should consider MRSA as a cause of unexplained illness in the elderly.
A member of the research team, Tim Peto, a professor of infectious diseases at Oxford University, said cleanliness in hospitals was important but more research was needed.
"Clearly, clean hospitals and washing hands are important... [but] I suspect, unfortunately, that you have to do more than that," he said.
MRSA (methicillin-resistant staphylococcus aureus) infection has increased in the UK over the last 10 years.
The bacterium can infect many sites, but a bloodstream infection - MRSA bacteraemia - can be one of the most serious forms.
Faulty immune system?
The University of Oxford researchers say the national surveillance scheme, which counts MRSA bacteraemia by hospital trust, has not yet addressed whether cases of MRSA bacteraemia are arriving in hospitals from the community.
They looked at MRSA and methicillin sensitive Staphylococcus aureus (MSSA) bacteraemia in patients on arrival at the John Radcliffe Hospital, the Radcliffe Infirmary - which operate as one teaching hospital - and Churchill Hospital, all in Oxfordshire - between 1997 to 2003.
At the teaching hospital, patients admitted from the community accounted for 49% of total MSSA cases and 25% of total MRSA cases.
Most patients (at least 91%) admitted with MRSA bacteraemia had previously been in hospital, half had never had MRSA detected before, and 70% were admitted to emergency medical and surgical services.
A similar pattern was observed in the district hospital.
The suspicion is that the patients, who on average had last been in hospital 46 days before their readmission, had been colonised with MRSA during that first stay.
But the infection only took hold while they were away from hospital - perhaps because their immune system was not working well.
Dr David Wyllie, clinical lecturer in microbiology at the University of Oxford, who worked on the study, said: "We do not think these patients had the strain of MRSA which has been seen in the community because the pattern of antibiotic sensitivity was typical of hospital-acquired strains."
He said such infections could be difficult to spot because patients would have symptoms such as fever, confusion and low blood pressure - which can also signify illnesses with other causes.
But he said doctors needed to realise MRSA could be one of those causes - even if the patient had not recently been in hospital.
"Doctors need to consider whether they should be giving antibiotics effective against MRSA to elderly patients coming back into hospital who are seriously ill with infections.
"In these patients, MRSA is not generally considered."
Professor Curtis Gemmell, director of the Scottish MRSA Reference Laboratory, said: "The Oxford researchers haven't categorically shown these are not cases of the community-acquired strain of MRSA, because they have not done a molecular examination."
But he added: "A lot of the patients identified as having MRSA on admission to hospital were elderly. And there is a lot of movement of elderly patients between hospital, nursing homes and other healthcare facilities.
"And that's where they are likely to pick up MRSA.
"They are likely to be at a higher risk. But I don't think that message has got through to hospital doctors."