Yet more authorial ignorance: yeast and mould are both fungi. Additionally, they are naturally found… even in the air.
On the plus side, they acknowledged that not all diseases are caused by microbes! Overextending one cause (together with over simplification) has caused some of the worst cases of medical ignorance in history — to wit, the classic form of the theory of the Four Humours. (One of a prominent group of cases where in ancients had a far better understanding than people give them credit for. The original theory of humours was an Ancient Near Eastern theory that some illnesses were caused by having too much or too little of necessary substances, and that such imbalances could also affect behaviour. Its basically a very rudimentary understanding of nutrition and hormones — which is why it sounds so modern. It was later first simplified to a mere four substances within the body — most likely to match up with the classical elements, another example of the same ilk — and then extended to be the cause of all illnesses.)
Also glad that they mentioned expectations of behaviour when being guarded. Especially for children of aristocrats, who should obviously know better. That never comes up in stories.