The study reported here examined the effects of experimentally induced minor illnesses (colds and influenza) on the efficiency of human performance. Influenza impaired the ability to detect and respond quickly to stimuli appearing at irregular intervals, but had no effect on a task requiring hand-eye coordination. In contrast to this, colds impaired hand-eye coordination but had little effect on the detection tasks. These results are of great practical importance because many skills clearly involve both attentional and motor factors and are, therefore, likely to be impaired by minor illnesses. The findings are also of major theoretical interest because of the dissociation of psychological functions produced by the different types of illness.