Measles outbreaks were all but eliminated in the US as of the year 2000. How did we get here? The answer is simple: diseases can regain a foothold when we take for granted the very means we have to stop them.

This is precisely why we need to start thinking about public health issues in a global context.

Diseases don’t care about borders. This is one lesson Ebola has surely reminded us of over the last several months. The notion that public health can be an exclusively national conversation is dangerously old-fashioned in 2015. Disease prevention will always be a global issue in today’s connected world. The fact that the CDC has attributed the current US measles outbreak to American travelers visiting the Philippines is but one proof point of this.

Where I work, at the Gates Foundation, we’re unequivocal in our stance on vaccines. They are about the closest thing we have to a miracle in modern medicine. Vaccines are cheap, highly effective, and—above all—they save lives. Millions of lives. The measles vaccine alone has saved more than 15 million lives since 2000.

Unfortunately, the remarkable success of vaccines is part of the problem. They’ve been too successful. As Melinda Gates perfectly summarized recently: when you forget what death from measles looks like, it is easy to take vaccines for granted.

I traveled to Ethiopia last year and saw firsthand the lengths to which parents go in order to get their children vaccinated. In places where there are still devastating, deadly epidemics of measles or meningitis, parents simply aren’t worried about the same things as we are in the US. They want that vaccine. Often they are willing to walk for miles, stand in line for hours, and endure immense heat in order to get them.

All of which makes it all the more startling to see that 16 African countries have now overtaken the U.S. in the race to vaccinate against measles.

The larger medical science issue here is that vaccines work best when everyone gets them. The more people vaccinated within a community, the more protection against disease is provided to the entire group. This herd immunity fortifies entire populations against disease, including babies who are too young or individuals who are medically unable to get vaccines.

But this shared immunity is compromised in poorer countries where vaccine coverage is inadequate, just as it erodes here in the US when people opt out of receiving vaccines. When it works, herd immunity a beautiful thing. It’s not often a medical treatment not only benefits an individual child, but entire communities, and future generations too.

Make no mistake: until we eliminate diseases like measles everywhere, they are a threat to us anywhere.

There is much work to do on that front, but I still see reasons to be optimistic. I’m encouraged to see concerned parents speaking up, and conversations like #IAMTHEHERD bringing science and a sense of shared responsibility into the debate. New supporters, such as artists, are standing up to help reach more people with the facts. (I’m particularly a fan of the comic below. Have a look and you’ll pretty quickly see why.)

Most of all, we should all feel encouraged when the question of whether or not to vaccinate is a remnant of the past once again—and when we’ve finally stopped taking life-saving vaccines for granted.

Art by Maki Naro, follow him on Twitter at @ScienceComic.