Iron deficiency and vitamin A deficiency reduce immunity function. Resistance to infection increases greatly with age. Malnourished children aged two years plus have greater immunity than younger better-nourished children. Well nourished exclusively breast-fed infants below four to six months can be expected to be relatively healthy. Infant and young-child morbidity is highly seasonal.
Different regions have distinctive seasonal patterns.
Infants selected for comparison in positive-deviance studies should be matched both for age and season of the year in which the data are collected. 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.