Identifying and understanding the causes of death and disease is key to improving health world-wide. But establishing and quantifying cause-effect relationships is difficult. Suppose that a large number of HIV-positive patients are prescribed one of two drugs: a well-established drug, and a new competitor. If more patients on the new drug develop AIDS, can we conclude that it is inferior? What if the new drug was prescribed to more seriously ill patients? Indeed, the decision as to which drug to prescribe is likely to be influenced by a patient‘s CD4 count, a measure of immunity and hence disease severity. But the drugs work precisely by increasing CD4 count. Thus CD4 count affects the choice of drug and vice versa. This so-called causal feedback makes drawing causal inferences even more difficult; but methods have been developed to address this. This Fellowship aims to extend and apply these methods to address three important questions, each affected by causal feedback, in low- and middle-income country settings: (1) what is the causal effect of diarrhoea on stunted growth? (2) among HIV-positive individuals, what is the causal effect of antiretroviral therapy on hospitalisation? (3) what is the causal effect of maternal fertility on child mortality?