THE PRIORITY OF B VACCINATIONS IN CANADA The increase in the number of children attending schools in Canada and the potential for transmission of viral infection in that environment is a problem of great size. A virus is defined as "a morbid principle, or a poisonous venom, especially one capable of being introduced into another person or animal" in The Shorter Oxford English Dictionary. Due to their miniscule size, it was not until 1885 that Adolf Mayer observed their existence in the mosaic (leaf-spot) disease of the tobacco plant, which ....