THE LESSON history keeps teaching us is that we never learn our lesson. Fear, ignorance, and prejudice inevitably get in the way. In the winter of 1885 an outbreak of smallpox in Montreal claimed more than 3,000 lives - 85 per cent of the victims were under the age of 10, 66 per cent were under the age of five. It was "the last epidemic of smallpox to devastate a city in the Western world" and it was completely preventable. Michael Bliss`s Plague: A Story of Smallpox in Montreal (HarperCollins, 306 pages, $26.95 cloth) is a detailed chronicle of the series of blunders, misunderstandings, and irresponsible behaviour that resulted in the spread of a disease for which an effective vaccine had been found almost a century earlier. On the dust-jacket of Plague there are the usual misleading blurbs about this account being "a non-fiction novel." It`s not, even though Bliss has written large sections of the book in the present tense. But Plague is a compelling popular history of an episode relatively unknown even to Montrealers. One of the most interesting subplots explored by Bliss is the conflict that raged between those in favour of vaccination and those against it. The forces of antivaccinationism proved to be self-righteous, fanatical, and unfortunately influential. Of course anything that happens in Montreal is bound to be coloured by language tensions -more than 90 per cent of the victims were French-speaking, for complex reasons that were mainly class-related - and the smallpox epidemic was no exception. It caused riots and military intervention, turning the city`s two communities against each other, making an already tragic situation worse.