Understanding a problem may help to solve it - but can also reveal it to be tougher than anticipated. So it is with Staphylococcus aureus, the Golden Staph: a never-ending headache for infection-control staff in hospitals, and responsible for the disruption caused by ward closures when it gets out of hand.

The conventional view of S. aureus, says Dr Nick Day of the department of tropical diseases at Oxford University, has been that it is an opportunist. Vast numbers of us harbour this common microbe on our skin or inside our noses. Only in hospitals does it create significant problems - and That is because hospitals are full of people in a poorly condition whose immune systems are less able to keep the bug at bay.

A study by Dr Day and colleagues (Science, 2001, 292: 114) has undermined this view. Using methods developed in Oxford, they analysed the genotype of strains of the bacterium taken from patients with hospital- and community-acquired disease. They then compared these strains with others taken from the noses of disease-free individuals.

The findings reveal that most illness caused by S. aureus is attributable to just a few of the many strains doing the rounds. In addition, these same strains turn out to be the most successful colonisers of the human nose.

And, to make matters even worse, they are also the ones which tend to acquire the genes conferring multiple resistance to antibiotics.

As a House of Lords select committee pointed out recently, resistant strains now appear to be endemic in virtually all hospitals.

In short, it is really no surprise that the Golden Staph is such an intractable problem. It may indeed be taking advantage of weakened patients - but it is also genetically geared up to do so. Dr Day's hope is that once the genes conferring these fiendish properties have been identified, they can be attacked. Managers trying to cope with closed wards will hope he is right.