Most studies have found that the incidence of infection does not differ significantly between the well-nourished and the malnourished, but the severity and duration are greater for the malnourished. Diarrhoeal infection is much more strongly associated with poor growth than respiratory or other types of infection. The effect of parasites on growth status depends on the degree of infestation, with light worm loads having almost no effect, but heavy worm loads being quite serious, for example. Age of exposure determines the virulence of some infections. Measles and pertussis are both believed to cause higher rates of malnutrition and mortality in Africa than in the Asian subcontinent, for example, because age of incidence is significantly lower in Africa.
Children lose their appetite and almost invariably lose weight when they are sick.
Exceptions to this rule are very minor colds, and skin and eye infections. During recovery the child's appetite increases and its catch-up growth rate may be up to 19 times the reference rate, depending on its degree of depletion and the nutrient density and quantity of the food it receives. Protein requirements increase during recovery because the stress of illness catabolizes protein stores. For this reason, positive-deviance studies may wish to weigh infants frequently and monitor their diets during illness and convalescence. Morbidity recall for a longer period than the two weeks prior to interview is not considered reliable.