Mary received a flu vaccine. Why didn't the vaccine make her sick?

Mary didn't get sick from the flu vaccine because it contains the smaller portion of the flu which isn't strong enough to infect you. Moreover, your white blood cells attack the portion of the flu while the memory cells take note of the flu, so when it comes next time, it won't affect your body because it knows how to fight the virus.

Hope this helps!

organ system-an organ system is a group of organs that work together to perform one or more functions. each does a particular job in the body, and is made up of certain tissues.