Optimal air distribution is pivotal for managing airborne infection risk in infectious respiratory diseases such as COVID-19. While existing studies evaluate and contrast the effectiveness of various air distribution systems in reducing airborne infection risk, the underlying mechanisms through which air distribution regulates these risks remain unclear. This chapter explores the mechanisms by which air distribution mitigates both overall and local airborne infection risks. Experimentally validated CFD models simulate contaminant concentration fields in a hospital ward, forming the basis for evaluating COVID-19 airborne infection risks using a dilution-based extension of the Wells-Riley model. Diverse air distribution systems—including stratum ventilation, displacement ventilation, and mixing ventilation—at different supply airflow rates are assessed. Findings reveal that variations in overall and local airborne infection risks across different air distributions and supply airflow rates exhibit complex and nonlinear trends. Contaminant removal and contaminant dispersion are identified as the respective mechanisms for overall and local airborne infection risk control, applicable irrespective of airflow patterns or supply rates. A strong contaminant removal capacity enhances overall risk control, as demonstrated by a 0.96 coefficient of determination between the contaminant removal index and the reciprocal of overall airborne infection risk. A high contaminant dispersion capacity improves local risk control, with a 0.99 coefficient of determination between the contaminant dispersion index and local airborne infection risk.