Hospital superbugs come in several forms. The bacterium which has made the biggest headlines in recent years is staphylococcus aureus, which is carried by 10% of the population in the nose and on the skin. Outside the body, the bacteria are harmless. But they can cause serious blood poisoning inside the body.

Now resistant to penicillin, the next best defence against it is the antibiotic methicillin but resistant strains are becoming increasingly common.

The most infamous, and the first to be given the "superbug" label is a class of bacteria called methicillin-resistant staphylococcus aureus (MRSA).

This is treated by the antibiotic vancomycin. But cases of partial resistance to this antibiotic do crop up.

Methicillin-sensitive staphylococcus aureus (MSSA) can sometimes be treated with methicillin. This is the bug Leslie Ash caught at Chelsea and Westminster Hospital last year.

Both MRSA and MSSA infections are most likely to take hold of weak patients. The attributed death rate for MSSA is 8%, compared with 21% for MRSA.