One of the first major crises in the book is Cullum arriving, with the news that the Bengal Plague as pretty much wiped out the Struan family in Scotland:
"Plague came to Glasgow in June," Cullum said dully. "They say it came by ship again. From Bengal - - India. First to Sutherland then Edinburgh, then it came to us in Glasgow. Mother's dead, Ian, Lechie, Grandma - - Winifred's so weak she won't last. Grandpa's looking after her." He made a helpless gesture and sat on the arm of the sea chair. "Grandma's dead. Mother. Aunt Uthenia and the babies and her husband. Ten, twenty thousand died between June and September. Then the plague disappeared. It just disappeared."We know the Bengal Plague as Cholera. (And, fortunately, few of us have first-hand knowledge. It's nasty.) Cholera is pretty much a water-born disease. You don't catch it by just being close to a sick person. You have to ingest (eat or drink) the germs.
Here's basically how it was spread. Sick Person picks up the bug somewhere, and goes home. He is very ill with diarrhea, and the family nurses him. Family takes the basins, chamber pots, clothing, rags, sheets, etc, the sick person used, and washes them and dumps the wash water in the sewer. The sewer water goes back into the drinking supply, and voila, anyone using that water drinks a big supply of cholera germs. The whole process starts again, but on a larger number of people. You can read a very detailed explanation of this in the excellent book Ghost Map, which describes how cholera was stopped in a London outbreak in the 1850s.
A bit later in the book, Cullum is disgusted by his father's insistance on cleanliness - thinks wiping and handwashing afterwards is a waste of time, and that washing his clothes will "spoil the cut." How little he knew!