Maggie probably got it from the Swartalves kids as asymptomatic carriers, and gave it to Harry. She probably had little sign as her magic hasn’t come in yet, so she may have had the equivalent if a cold just snot, no ectoplasm. Maggie hadn’t started her new school yet.
This may mean Maggie and Harry got a different strain to that normally encountered by human children, a variant mutating, jumping from Swartalves to humans, which is how the disease probably began in the first place as a disease of some supernatural race which jumped the species barrier. It probably just happened again, as unfortunately these things do.