How do vaccines prevent the spread of some infectious diseases?

It activates the body's natural defenses and puts the body on "alert"

answer #2=vaccines help by bringing forth white blood cells to the invading area faster than regular. It also injects antibodies into your body to help kill the bacteria and viruses.

Answer #3=

You have to remember (talking to answer number two) that viruses are not considered alive so you actually dont "Kill them" you dispose of them or get rid of them! :)