There are currently heightened public concerns over infection rates in UK hospitals, and in particular those caused by so called ?superbugs? that have become resistant to available antibiotics. One such bug is Clostridium difficile. It causes debilitating diarrhoea, which in extreme cases can kill. It mainly affects the elderly. As this proportion of the population is increasing, the disease is becoming more common. Worryingly, a new, even more deadly variant has now arrived in Europe from North America. Aside from the human suffering, it costs the NHS over #402 million per year, and now is responsible for more deaths per year than MRSA. To control infections, we need to understand how an organism causes disease. Under Wellcome Trust sponsorship, the complete genome sequence of the organism (ie., its genetic blueprint) has been determined. However, whilst we now know the sequences of every gene in the C. difficile chromosome, we do not understand what they are doing. The best way of working out what genes do, is to mutate them (make them non-functional) and assess the consequences. Until now this has not been possible. We have now developed the tools needed, and wish to use them to better understand how this bug causes disease. This should eventually lead to better ways of controlling the disease.