As more and more parents are choosing not to vaccinate their children or are vaccinating them later, diseases like measles are making a comeback. +It's Okay To Be Smart break down the history and science of vaccines.
The US eliminated measles (or so we thought) in the year 2000, but because of parents not vaccinating their kids, or doing it later than recommended, it's making a comeback. This is something simple to prevent with vaccination. Get this though… in a susceptible population measles each sick person will infect up to 18 others. For comparison… ebola the infection rate is 2. 90% of unprotected people who come in contact with measles (even just breathing the same air) will become infected… but it can be easily prevented with vaccination.
For more information click on through to the YouTube link, they have tons of references and further reading.