The measles outbreak is spreading. This year, there have been over 500 cases in the U.S. alone. Dr. Paul Sax is the clinical director of infectious diseases at Brigham and Women's Hospital and a professor at Harvard Medical School. He spoke with WGBH All Things Considered anchor Barbara Howard about the spread of measles. This transcript has been edited for clarity.

Barbara Howard: Those of our listeners who are old enough to recall when measles was kind of a right of childhood, a right of passage — very few of us can recall friends or classmates actually dying of the virus. So is the worry overblown?

Dr. Paul Sax: It’s true that measles, in general, lasts a week or two and people recover from it uneventfully. But there have always been people who have developed very severe measles with significant complications and even deaths. The most common complication that leads to death is pneumonia. Measles pneumonia can be very severe. In addition, it can infect the brain and cause swelling of the brain, something called encephalitis. There are children who developed irreversible hearing loss from ear infections. It can affect sight. The worry is not overblown at all.

Howard: Well we've had what, more than 500 cases so far this year. How does this compare to years past?

Sax: So this is substantially worse than the last several years. The most severe year previously in the last decade was 2014, when there were just over 600 cases, and we're almost certainly going to go right past that. If a person with measles enters a room and there are people there who have not been vaccinated, there's about a 90 percent chance that it will be spread to another person. But I think it's important to look back in the early 2000s, when we really had control of measles in this country, and there was actually very little transmission happening at all.

Howard: In the year 2000, it was thought to have been eradicated.

Sax: Exactly, and of course our borders are not completely tight, there are people who are going to be traveling. But if we have enough of what's called "herd immunity" in the community, those single cases will just stay that — single cases.

Howard: There was a case of a young man, 26 years old, undergoing chemotherapy, being treated for leukemia. Talk about that.

Sax: There are people who can't get the measles vaccine because their immune systems are too weak or no longer respond to it because their immune system had been weakened medically. So there was a young man, he developed a very severe pneumonia that initially the clinicians couldn't figure out what it was. Ultimately, he developed the rash. They recognized that it might be measles, and he ended up dying of measles. Now, he was immunized. The important thing for people to understand is that people whose immune systems are weakened by cancer chemotherapy or other reasons, they can still get very severe measles. So one is getting immunized to protect not just yourself, but to protect others.

Howard: So anyone who is being treated with chemotherapy is vulnerable to measles?

Sax: Absolutely. People treated with chemotherapy, people receiving treatments for rheumatoid arthritis, Crohn's disease and forms of colitis. Many people are receiving immuno-suppressive drugs, and they are the ones at the biggest risk of getting complications from measles.

Howard: And of course there are babies who are under the immunization age.

Sax: The extremes of age are always more vulnerable. There’s some evidence that pregnant women who are not immune may have more severe measles. So there are many severe complications of measles that people really are not aware of anymore.

Howard: Prior to the vaccine for measles, how common was it to have those kinds of complications?

Sax: There would be several hundred measles deaths a year in the United States. Since the immunization has come along though, that number has dropped precipitously.

Howard: When you talk about herd immunity, what percentages are we talking about?

Sax: So with measles, because it is so contagious, we need about 95 to 97 percent of the population immunized. That's a very high percentage, but you know, it's doable. I want to say one thing about the United States — we have very strong vaccine regulations for school entry. That's one reason why our measles outbreaks in the United States are much smaller than those in Western Europe. We should not weaken the regulations about requiring vaccines for entering schools. These so-called non-medical exemptions really should be eliminated. That's what caused the measles outbreak in the Pacific Northwest — more parents were taking these non-medical exemptions to vaccines. And recently I was very heartened to see that they strengthened the regulations.

Howard: I try to imagine being a clinician, and science tells you that this is a worthwhile thing. And yet people who want to do right by their kids and keep them well-fed, give them wholesome food, are skeptical of this [vaccinating their children]. Does it frustrate you that they'll turn to un-proven things, non-FDA regulated things, but they won't do this?

Sax: It is frustrating. On the other hand, one can understand that they want what's best for their children, just that they've been misled by false information. It's just, you sort of strike a fine line between wanting people to do the right thing for both themselves and for the community. And that's really what we're talking about here.

Howard: Well what happens if you belatedly want to get your child immunized? Is that window only for childhood, or can you get immunized later?

Sax: If a child has not been immunized, there’s no time frame which is inappropriate to actually get the vaccine.

Howard: There's no window for getting vaccinated, it's not too late?

Sax: It's not too late.

Howard: That's Dr. Paul Sax, clinical director of infectious diseases at Brigham and Women's Hospital and professor at Harvard Medical School.