I think there's zero chance that all of the following are true:

1) Measles is a deadly disease caused by exposure to a single microorganism too small to be imaged (at all, ever) by a light microscope.

2) People who are sick with measles produce trillions of individual measles-causing microorganisms and spread them into the environment.

3) Despite (1) & (2), we're not all already dead.

4) Despite (2), the measles-causing microorganism has never been isolated from a sick host.

5) Despite (4), somehow an injection was developed that uniquely protects against the measles-causing microorganism.

6) The injection in (5) is completely safe.