Zhang D et al. showed that children with upper respiratory tract infections who were <3 years old and had an intestinal microecological imbalance had lower values of CD3+ and CD4+ than those in the non-disordered group and the healthy control group, and children with upper respiratory tract infections who had an imbalance of the gut microbiota at an age of less than 6 years old (including the <3 years old and the 3–6 years old age groups) had reduced levels of IgA compared with those of the non-disordered group and the healthy control group at the same age. Here, CD4 is linked to Recurrent upper respiratory tract infections.