It was shown many years ago that viruses can cause common colds but it is often forgotten that there were prolonged investigations and disagreements before this was agreed. It was difficult to detect or propagate any virus in the laboratory and it has taken more than two decades of research to arrive at a reasonably complete list of the viruses that cause colds and related diseases. To do so, new techniques were developed by which these very specialized viruses can be grown and studied. Nevertheless unanswered questions remain on the relation between them and the diseases. This research has required finding answers to several different questions about the viruses and the diseases. The questions cannot all be answered in the same way for all virus infections, and recent research has shown that we can learn a great deal about certain viruses and diseases without answering all of the questions; for instance, the role of rotaviruses in diarrhoea was established though the viruses could not be effectively grown in the laboratory. In the light of these ideas I shall refer to the evidence that mental diseases may be due to virus infections and the recent finding that cerebrospinal fluid from such patients produced a cytopathic effect in tissue cultures that was apparently due to a virus-like agent.