Giardia: both a harmless commensal and a devastating pathogen.

The highly prevalent protozoan Giardia lamblia is an enteropathogen that can be asymptomatic in some individuals, while leading to persistent diarrhea and substantial morbidity in others. In this issue of the JCI, Bartelt et al. describe a mouse model of the disease and investigate the contribution of coincident malnutrition with the development of symptomatic infection. This work in part explains how Giardia infection can lead to growth retardation, and may offer insights that guide future therapeutic strategies.