Although most vaccine-preventable diseases are rare in Canada due to mass immunization, they are common in other parts of the world and can be reintroduced by travelers who carry them from country to country. When immunization rates drop, diseases come back. Improvements in hygiene and sanitation over the twentieth century have indeed improved our health, but not without the direct impact of immunization. When vaccines are introduced, disease rates plummet.
Since we do not see these many of these diseases today, it is easy to forget that in the past thousands of people suffered and died from childhood diseases such as measles, diphtheria, whooping cough (pertussis) and polio. Even today these diseases can lead to pneumonia, choking, brain damage, heart problems, blindness, disabilities and death in children who are not protected.
Public Health Agency of Canada: Vaccine-preventable diseases and surveillance
National Advisory Committee on Immunization: Canadian Immunization Guide