A single doctor, nurse or physiotherapist with dirty hands can cause a widespread hospital infection, even when all other staff diligently scrubs their hands, new research finds.

The study found that a single contaminated worker who roams from patient to patient can play a disproportionate role in spreading pathogens such as MRSA (Methicillin-resistant Staphylococcus aureus).

In fact, a single traveling nurse, physical therapist or food worker who failed to properly wash his or her hands after each patient visit could create a rate of infection that would be about the same as if 23 per cent of all the other staffers failed to wash their hands.

Researchers at France's National Institute of Health and Medical Research or INSERM used a mathematical model to better understand how hospital-based infection outbreaks can happen.

Laura Temime and colleagues at INSERM used a mathematical model and analyzed a hypothetical intensive care unit. The researchers separated healthcare workers into three groups:

• a "nurse-like" group, who made frequent visits to a small number of assigned patients
• a "doctor-like" group, which made infrequent visits to a larger number of patients
• a "peripatetic" group - workers such as a physical therapist or radiologist -- who visited lots of patients daily.

The model tracked what would happen over a month if a single infected patient were introduced into an 18-bed ward, and then the different workers failed to follow hand hygiene rules.

When all health care workers were compliant, the model predicted between 1.5 and 5.8 new cases over the month, depending on how transmissible the pathogen was.

The size of the outbreak increased if a single worker neglected hand hygiene -- to between 1.7 and 6.8 cases on average over the month. But the results varied depending on which workers neglected their hygiene, the researchers found.

One peripatetic worker who failed to wash his or her hands and who traveled about the hospital treating many patients could greatly raise the hospital infection rate, becoming a "superspreader," they calculated.

Infection rates increased by up to three times more when a "peripatetic" staffer failed to wash his or her hands compared with a nurse assigned to one ward, for example.

"Our findings may explain several reports of outbreaks that were traced back to peripatetic healthcare workers," they wrote, adding that such workers should be targeted for reminders about hygiene measures in hospitals.

The results appear in the Proceedings of the National Academy of Sciences.