Adenovirus is one of the viruses that causes the common cold. No need for immigrants to bring it in.
WebMD:
“Adenoviruses are a group of common viruses that infect the lining of your eyes, airways and lungs, intestines, urinary tract, and nervous system. They’re common causes of fever, coughs, sore throats, diarrhea, and pink eye.”
Therefore, since adenoviruses are common, and what those kids are having is not common, then whatever they have is new. And nationwide. Just so happens that recent volumes of immigration is new and nationwide.