 Cotrimoxazole is a synthetic antibacterial combination that blocks two steps of folate metabolism involved in the biosynthesis of nucleic acids and proteins essential to many bacteria and some parasites, including Plasmodium falciparum. It is cheap, widely available and has an established safety profile in African populations. Cotrimoxazole prophylaxis dramatically reduces mortality among children with HIV, irrespective of the degree of immune suppression. The primary effect is in reducing bacterial infection, especially pneumonia. the effect has been demonstrated in areas with high levels of cotrimoxazole resistance bacteria. It is also widely used in developed countries among children with other immune deficiencies to prevent infection. Children with severe malnutrition are immune deficient, as evidenced by their susceptibility to infectious diseases, and may therefore benefit from daily antimicrobial prophylaxis. 