In multivariable Cox regression analysis of time to subsequent cryptosporidiosis, which included demographic, socioeconomic, and infant anthropometric variables, Cp23 IgA in the upper 50th percentile was associated with statistically significant reduction in hazard ratio of 24% (95% confidence interval, .5%–42.3%; P = .046). The gene discussed is CD79A; the disease is cryptosporidiosis.