Minor illnesses, such as colds and influenza, are frequent, widespread and a major cause of absenteeism from work and education. Yet the clinical symptoms of such illnesses may not be so great as to stop people from working or from carrying out everyday activities. It is therefore important to determine whether these viral infections alter central nervous system function and change performance efficiency. Data on this topic are almost non-existent, which in part reflects the difficulties inherent in carrying out such studies. In real life it is almost impossible to predict when such illnesses will occur, and difficult to establish which virus produced the illness. This problem was overcome by studying experimentally induced infections and illnesses at the Medical Research Centre (MRC) Common Cold Unit in Salisbury. Results from this research programme show that: (i) colds and influenza have selective effects on performance; (ii) even sub-clinical infections can produce performance impairments; (iii) performance may be impaired during the incubation period of the illness; (iv) performance impairments may still be observed after the clinical symptoms have gone. These results have strong implications for occupational safety and efficiency and it is now essential to assess the impact of naturally occurring colds and influenza on real-life activities.