I had a COMPLETELY Different take on The Conjouritis. I thought it might relate to Maggie and HER powers. NOT Fun Fact: childhood diseases are sometimes worse in adults (chickenpox is), Maggie could have a less noticeable form of the same disease, Harry is bigger and stronger with more magic, so it's more disruptive. Here's the thing about children, the spread germs VERY efficiently, they bring cold from school and EVERYONE gets sick! Being a parent often means being sick because your kid gave you a cold, sometimes your cold is worse than theirs. If you missed a childhood disease in your own childhood, your own child could bring it home from school or from friends. Maggie goes to school and had been shown interacting with children in the magical community (the Svartalves in this novel) so she could have brought something home from any of the places she goes and the kids she plays with. Dresden has become a full time parent, sometimes it's gross, and sometimes it makes you sick.  However I do concede that my mind might be on children spread things in school and bringing them home only because of the "Summer of COVID."