Through weekly data and stool samples and routine blood tested for rotavirus IgA and IgG from 200 Mexican children from birth to the 2nd birthday, that model cohort showed that first natural rotavirus infections are the most severe, but incremental and heterologous infections confer protection that typically leads to reduced severity—a concept now recognized as the operating principle for how the currently effective live-attenuated rotavirus vaccines work. Here, CD79A is linked to infection.