Such feverish children in the Atlanta area are part of a pattern of viral infections that pediatricians, infectious-disease specialists and epidemiologists have noticed cropping up this spring in the United States, especially in the South. These common viruses are showing up at a distinctly uncommon time of year — and sometimes with uncommon virulence among children whose immune systems did not begin building up familiarity with them while the pandemic kept people isolated at home. The comeback of ordinary viruses is widely regarded as a dark underside of a season in which the coronavirus has been receding in much of the nation as vaccinations provide protection. As a result, people are shedding masks and abandoning social distancing — and resuming spreading viral droplets. Last week, the Centers for Disease Control and Prevention issued an advisory warning that respiratory syncytial virus, known as RSV, has been detected with increasing frequency since March in 10 Southern states, plus Texas, Oklahoma and New Mexico. RSV is the most common cause of bronchial infections and pneumonia in children under age 1, with a season that typically runs roughly from November to early spring.