RNA polymerase enzyme from Norwalk virus, molecular model. This enzyme makes RNA (ribonucleic acid) from the RNA template within the virus, thereby helping the virus replicate itself within a host cell. The Norwalk virus is transmitted via contaminated water and food, and causes vomiting and diarrhoea. In this model, the solid colours represent the secondary structure of the protein (spiral ribbons are alpha helices; arrows are beta sheets), and the faded tubular structure behind represents the individual atoms and bonds.