The distribution of worm burdens in human populations is a major determinant of both the dynamics of transmission and the level of community morbidity. The distribution exhibits convexity with host age, which appears to correlate with exposure in the young age-classes but not in adults, and may be evidence for the development of an acquired immune response. The distribution between individuals is typically overdispersed. Individuals are predisposed to high (or low) intensity of infection and to a correspondingly high (or low) rate of acquisition of infection. A major epidemiological question is whether this reflects individual differences in environmental exposure or susceptibility. Environmental studies that have observed clustering of intense infection in particular households are supportive of either mechanism. Individual host behaviours that predispose to infection have an overdispersed distribution and may alone, or as compounding factors, generate the observed distribution of infection intensity. Factors such as host nutrition and physiology may modify host immune-responsiveness and hence susceptibility. Preliminary evidence suggests correlates between infection intensity and HLA class II antigens, and tentatively implies a genetic factor in susceptibility. These findings suggest that further understanding of the relative importance of environmental factors and resistance to the acquisition of intense infection is dependent upon a multidisciplinary approach to epidemiological field study.