PURPOSE OF REVIEW: There is growing concern that changes in nurse workforce and hospital-restructuring interventions negatively impact on patient outcomes. This review focuses on the association between understaffing and health-care-associated infections. RECENT FINDINGS: There is a large number of studies showing that overcrowding, understaffing or a misbalance between workload and resources are important determinants of nosocomial infections and cross-transmission of microorganisms. Importantly, not only the number of staff but also the level of their training affects outcomes. The nurse workforce is ageing, mainly due to fewer individuals' engaging in a nursing career. This phenomenon, combined with cost-driven downsizing, contributes to a nursing shortage, and this tendency is not expected to revert unless important system changes are implemented. The causal pathway between understaffing and infection is complex, and factors might include lack of time to comply with infection control recommendations, job dissatisfaction, job-related burnout, absenteeism and a high staff turnover. SUMMARY: The evidence that cost-driven downsizing and changes in staffing patterns causes harm to patients cannot be ignored, and should not be considered as an inevitable outcome. More research is needed to better define the optimal patient-to-nurse ratio in various hospital settings and to estimate the economical impact of the nursing shortage. All quality-improvement interventions should carefully take into account systems and processes to be successful, as the issue of staffing is essentially a structural problem.