Here in the US, a disease that was all but extinct has made a comeback.   I’m talking about measles.   It usually starts with flu-like symptoms and a raised rash on the face, followed by inflammation of the eyes, and then fevers as high as 40.5 degrees Celsius -- that’s 105 Fahrenheit -- and in the very young, brain damage, hearing loss, and even death.     The disease has been so rare in the U.S. that it was pretty easy for scientists to figure out where the outbreak started.   In December, a person carrying the measles virus visited Disneyland in California.   Now, more than 100 people have been infected, and it has spread to seventeen states.    Now, yeah, 100 people might not sound like a lot. But keep in mind that that is twice the number of cases the US usually sees in an entire year, and that the outbreak is only two months old.   Now the Centers for Disease Control is concerned that the outbreak could gain a foothold unless more people are vaccinated.    So how does something like this happen?    I’m Hank Green and this is SciShow News.   [Intro]   In the year 2000, scientists declared that measles had been officially eliminated from the United States.    Elimination, in a medical sense, means that a disease is no longer endemic, or regularly found in a certain region.     But eliminating the disease in the U.S. didn’t change the fact that it was still prevalent in other countries, like the Philippines -- where the current strain of measles seems to be from -- and it remains a leading cause of death among young children worldwide. Though, thanks to vaccinations, worldwide Measles deaths have decreased 78% in the last ten years.    Now, measles is a viral disease that’s communicable through airborne droplets of water, just like flu, and the viruses can linger on surfaces for a couple hours.    One infected person will infect, on average, 12 to 18 other people. That’s one of the reasons why this outbreak has spread so rapidly.   But of course, there is a vaccine for measles -- it’s been around for 50 years.    It uses a small amount of live measles virus that’s been weakened by a process called cell culture adaptation.   This process basically genetically modifies the virus by allowing it to grow inside of some kind of medium that isn’t human cells.   In the case of the measles vaccine we do this basically with, like, chicken embryos in a petri dish. After a while, the virus adapts to infecting chickens, and these adaptations make it less virulent to humans.   While a natural measles virus reproduces thousands of times during an infection, a weakened virus can only reproduce about twenty times inside of a human host.    But that’s just enough so that it doesn't cause any symptoms, but it does stimulate an immune response in our bodies, which allows us to produce antibodies.   Throughout the rest of our lives, those antibodies will be able to identify a measles virus so that our immune system can be awesome at destroying it.    But … the system isn’t quite perfect. 3 to 5 percent of the population can still contract measles even if they’re vaccinated.   And yet, if a high enough percentage of the population is immunized, the virus can’t spread.   Instead of each infected person creating twelve or eighteen new disease vectors, the average person creates less than one new disease vector, and thus, the outbreak just doesn't happen.   This, my fellow herd-members, is called herd immunity.    And it means that an entire community can/be protected, even if a few of those members aren’t immune to the pathogen.    In the case of the measles, 92 to 94 percent of a population has to be immunized in order for herd immunity to protect the whole group.    So, how do we know all this?   Well, a widespread vaccination program for measles, mumps and rubella -- all airborne viral diseases -- began in the US in 1963.    And as the country’s herd immunity increased, the incidence of measles went from 3 to 4 million cases a year in the 1950s down to just 37 cases in 2004.    But in recent years, immunization rates have began declining, due in part to the growing misconceptions that have erroneously linked these vaccines to autism and other disorders.   As a result, in nearly 500 public schools at ground zero for the current outbreak -- in Southern California -- the immunization rate has fallen below that 92 percent threshold, which has given the measles an opportunity to spread again.    But people who’ve contracted measles from this latest outbreak are all over the country -- more than half of them weren’t vaccinated.    Immunization rates in eight other states are even lower than California’s -- with Colorado coming in last, at around 81 percent. So this is probably not the last we’ve heard from measles.   Our herd is being compromised.   But, how did we get here?   How did we get to the point where people stopped using safe and effective protection against a disease, and decide instead to take their chances?   We’ll be talking about the science of why we sometimes ignore science, in a special infusion next week, so be sure to join us.   In the meantime, thanks for watching SciShow News. If you want to help us share science with the world, you can become a supporting subscriber at Subbable.com/scishow. And if you want to keep getting smarter with us, you can go to youtube.com/scishow and subscribe.