Malaria, measles, lower respiratory infections, and diarrheal illnesses are common pediatric medical problems that are often fatal in the context of extreme poverty. In nonpoor environments, however, these infections are controlled and managed in ways that minimize mortality. From a scientific perspective, genetic variation among microbes is a frequent and important component of their epidemiology, pathophysiology, treatment, and prevention. From a public health perspective, relatively simple measures can reduce the mortal effects of these diseases until successful vaccines become available and immunizations programs are established. Infants and children are especially vulnerable to poor outcomes from infections when undernutrition and other circumstances of poverty are present.