Something very disturbing happened recently at my school. It all began with an email:

And then there was a follow-up:

A few things to note: first, that Mumps is preventable by a vaccine: a 2-dose injection that is 88 percent effective. Second, Mumps is a very serious illness that can lead to severe complications. Finally, that both Simmons students who have Mumps were vaccinated. You may have never known a single person who has Mumps. Or, for that matter, Whooping Cough, Measles, Diptheria, smallpox, tetanus, polio, hepatitis A or B, rotavirus, rabies, typhoid fever, yellow fever, tuberculosis, or even Chicken Pox. Here's why: vaccines. Here's not why: it's not because these diseases no longer exist. It's because the vast majority of people in developed countries are vaccinated against these diseases. These diseases, which once killed thousands upon thousands of children in a single year, were once nearing extinction in developed countries -- which, by the way, means that there are no recorded cases, not that there are no pathogens left. And that, right there, is the crux of the problem.

The anti-vaccine movement is based largely around two principles: one, that vaccines cause autism and two, that vaccines are no longer necessary because these pathogens no longer exist. Both of those arguments are absolute horse poo, because there is not a single shred of scientific evidence to back up either claim.

Let's start from the beginning, though. How do vaccines work? As a second-semester nursing student at Simmons College, I feel qualified to explain this but you need not take my word for it. Check out this lovely website for proof that vaccines work by inciting a harmless and usually asymptomatic immune system response that allows your body to create specialized cell "weapons" against a very particular, very tiny organism that would otherwise give you, say, Tuberculosis and then, maybe death. The reason that a vaccine doesn't simply cause Polio or Tuberculosis is because the pathogen, the teeny weeny organism that gets injected into your body, is either weak, dead, or not even a whole organism. All your body needs is the antigen, the unique part of the pathogen that tells your immune system that this is not part of you and must be eradicated immediately. Your body then learns how to make cells that can completely disable and destroy that pathogen, and like an elephant, it never forgets (I mean, some vaccines need boosters so I guess that wasn't a great analogy but anyway). The next time that pathogen enters your body, your immune system whips up a bunch of those specialized ninja cells and saves the day long before you even knew there was a problem.

This stuff works, too. Below is a graph from the CDC showing the incidence, the number of cases of a disease, of mumps from 1986 to 2011. Look how close that line got to zero! Look at that!

So here's why you never hear about Smallpox or Tuberculosis cases: the pathogen gets destroyed before it's able to wreak havoc on the body it invaded. However, those pathogens still exist in our world. They are all over the place and if you don't have a vaccine, something much worse than one person getting polio can happen. Go back to graph from two seconds ago: do you see that huge spike around 2005? Let's talk about that.

When everyone who can be vaccinated is vaccinated, and some people cannot receive vaccines because their immune systems don't work properly, we have something called herd immunity. When nearly everyone is vaccinated against a pathogen, that pathogen never has the opportunity to reproduce and actually cause disease. When people stop getting vaccines, the pathogen now has the opportunity to multiply, to cause disease, and, worst of all for public health, mutate. A mutated pathogen is a very, very bad thing, because every time that pathogen reproduces, tiny errors are made in its DNA. And every once in awhile, that error creates a pathogen that is resistant to the antibodies produced in response to the vaccine. And that, friends, is why those two girls at Simmons got Mumps despite having the vaccine. They caught a mutated pathogen from someone else: someone who didn't get their vaccines, and thus became a breeding ground for vaccine-resistant Mumps. That person may not have been symptomatic. That person's body may have taken care of the problem before it actually became a disease, but bacteria and viruses multiply at exponential rates and would not take long for one vaccine-resistant pathogen to get out and cause a world of hurt.

Now run along, friends, and get your children vaccinated! You are not just endangering your kids. You are endangering us all. It sounds melodramatic, but it isn't. Pathogens are small and terrifying. Don't be fooled by their size, people. Oh, and the autism thing? The study that anti-vaxxers cite to "prove" a link between vaccines and autism? It was faked.