My sister-in-law is a pediatric intensivist. She had been working with a very sick little kids for 15 or so years when I moved her 6 month old niece and 2 year old nephew to London, and promptly started the eldest in nursery school. SiL, who had had all bugs in the US and hadn’t been ill for probably five years, got sick every single time we brought the children home for a vacation. There might of been some affectionate cursing at her brother and I back then.