Massive slums have become major features of cities in many low-income and middle-income countries. Here, in the
fi rst in a Series of two papers, we discuss why slums are unhealthy places with especially high risks of infection and
injury. We show that children are especially vulnerable, and that the combination of malnutrition and recurrent
diarrhoea leads to stunted growth and longer-term eff ects on cognitive development. We fi nd that the scientifi c
literature on slum health is underdeveloped in comparison to urban health, and poverty and health. This shortcoming
is important because health is aff ected by factors arising from the shared physical and social environment, which
have eff ects beyond those of poverty alone. In the second paper we will consider what can be done to improve health
and make recommendations for the development of slum health as a fi eld of study.