Living with uncertainty.

The persistence of highly endemic parasitic, bacterial and viral diseases makes individuals and populations vulnerable to emerging and re-emerging diseases. Evaluating the role of multiple component, often interacting, causes of disease may be impossible with research tools designed to isolate single causes. Similarly, it may not be possible to identify statistically significant treatment effects, even for interventions known to be effective, when multiple morbidities are present. Evidence continues to accumulate that nutritional deficiencies, bacterial, viral and parasitic coinfections accelerate HIV transmission. Inclusion of antiparasitics and other beneficial interventions in HIV-prevention protocols is impeded by reliance on inappropriate methodologies. Lack of full scientific certainty is not a reason for postponing safe, cost-effective measures to prevent irreversible damage.