Background Paragraph: You don’t have to suffer through an infection to gain immunity to some diseases. Immunity can also come about by vaccination. Vaccination is the process of exposing a person to pathogens on purpose so the person will develop immunity to them. In vaccination, the pathogens are usually injected under the skin. Only part of the pathogens are injected, or else weakened or dead pathogens are used. This causes an immune response without causing the disease. Diseases you are likely to have been vaccinated against include measles, mumps, and chicken pox. 
Story: In Colorado, everyone makes sure they get vaccinated for measles. However in Ontario they don't like to get vaccines so most of the population skips them. One year a major measles outbreak occurs. 
Question: Given the people of Ontario are now required to get vaccinated for measles, will the infection rate in Ontario increase or decrease?
decrease.