In some areas, immunity has been eroded so much that the child who's not vaccinated is now actually more vulnerable to the complications of infectious diseases.

I think that protecting children at the age where they're most vulnerable against diseases that are highly contagious is prudent.

We've been using vaccination in some form for hundreds of years now. We have almost nothing in our modern medicine that we've been using that long, and it's been consistently productive even though, you know, the older vaccines were much more dangerous than vaccines we're using now.

In the case of Pakistan, the CIA actually used a fake vaccination campaign to try to locate Osama bin Laden, so now vaccination is associated with espionage.

There's a cultural expectation that everyone will be immunized, in part to protect the entire population. When people refuse that expectation, they're indulging in a certain kind of political or social immunity.

Herd immunity is, it turns out, not incredibly easy to understand. It took me quite a bit of reading before I fully grasped it. But understanding herd immunity is essential to understanding why we vaccinate the way we do.