 Cotrimoxazole has striking protective efficacy against mortality among children with HIV, despite not altering the underlying immune deficiency. It is hypothesised that co-trimoxazole prophylaxis will have a similar effect in children immunocompromised because of severe malnutrition. Worldwide, severe malnutrition is commoner than HIV in childhood and co-trimoxazole is cheap and widely available, making it easily translatable to policy. 