The great Persian physician Abū Bakr Muḥammad ibn Zakariyyāʾ al-Rāzī, often described grandfather of pediatric medicine, was a meticulous man. Before the age of 30, he discovered ethanol thanks to the careful application of the then-new art of distillation. When overseeing the building of a new hospital in Baghdad, al-Rāzī hung raw meat around the city and broke ground where the meat putrefied most slowly. And, in one of the 200 or so books that he wrote, he created the first and most extraordinarily detailed account of one of the most infectious diseases ever known to man.

Since al-Rāzī first carefully documented it, this little strand of RNA tucked in a protein envelope has enjoyed a rare kind of notoriety, even in the shock-and-awe world of infectious diseases. In 1529, the Spanish introduced it to Cuba, killing two out of three natives. Over the next decade or so, the virus ravaged Central America, decimating many populations and killing up to half of all Hondurans. And in 1693 in colonial America, Virginia governor Edmund Andros issued a proclamation for a “day of Humiliation and Prayer” in the hope of waylaying the virus.

It’s not like that today, but the disease is no slouch either. In 2013, according to the World Health Organization, there were 16 deaths from the virus each hour, around the world, for the entire year. It is one of the leading causes of death among young children, despite our ability to safely vaccinate against it. It is estimated that between the years of 2000 and 2013, vaccination has prevented 15.6 million deaths.

Do you recognize it yet?

Everyone has, at some point, seen the picture of the measles kid — that straw-haired toddler covered in head-to-toe tomato-red splotches. It’s a photographic representation of abject misery: the poor child, betrayed by his immune system, with features blurred by a day four rash. But while a picture of hundreds of unsightly lesions might be shocking, it’s still not sufficient evidence for parents who more afraid of the false specter of Andrew Wakefield than the real-world danger of death by measles.

Despite the United States having declared measles eliminated in 2000, the virus has awoken anew, and with all the grace and predictability of a large bear coming out of sedation. Clusters of low vaccination rates are reviving this Lazarus virus of the West, with outbreaks springing up at a disconcerting clip. And though the Greatest Generation might know measles’ insidious tricks all too well, this virus is a bit too old-fashioned for many in younger generations to know much about it.

So, for anyone new to the disease, it’s time to meet your measles.

After all, the measles already knows you.

Though best known for its telltale dappled rash, measles is a wildly infectious upper respiratory disease. Like the flu, it’s airborne — and successful. It has a near-perfect infection rate: Put your baby in a room with a measles patient, and 9 times out of 10, measles is coming home with you. In the communal space shared between you and a coughing, sneezing measles-ridden asshole, the sweet oxygenated room air and unavoidable door handles are thought to remain infectious for up to two hours. And measles delivers a double whammy because a person becomes infectious before they even know they have it. Four days prior to the rash is when most people become able to spread the love. Here’s how the virus pulls it off.

So how do you know if you have measles? Symptoms start out like standard-issue wintertime gunk: fever, cough, runny nose, red eyes. A few days of that misery and the decorative stage of the illness starts with a carpet of red lesions that blossoms from head to toe. Koplik’s spots may appear before the rash — that’s when the bright, beefy red of the inner cheeks become studded with lots of tiny, blue-white dots. Complications from measles arise in almost one in three reported cases, and range from diarrhea (8 percent) and pneumonia (6 percent) to encephalitis (0.1 percent) and death (0.2 percent).

It gets worse.

After coming in on the air like an old-school miasma, the measles virus jumps in through the upper respiratory tract or the eyes. However, before it can make a beeline for the closest lymph nodes, measles has to get inside the cell. To let you in on a secret, the entire field of molecular biology is basically just the study of the secret handshakes between tiny bits of stuff, and that’s exactly what is happening here. The measles virus is encased in a membrane that has two different surface proteins: hemagglutinin (H), which is like a magic wizard lock picker, and fusion (F), which clearly has the better name. The virus rolls on up to an epithelial cell and mashes its magical lock picker into a receptor site; slip it in the right place and it kicks off a molecular chain of events that wakes up the fusion protein. The fusion protein then blorps the virus into the cell. It’s all very technical.

Once things get started, measles really gets around the body. Traveling from the lymph tissue near the site of infection, it hitchhikes over to some nearby lymph nodes and sets up shop for replication and destruction: Part of this process involves a steep reduction in the body’s available cache of certain types of infection-fighting white blood cells. Copies of the virus then hop into the bloodstream (viremia), and from there it’s a one-way ticket to measles town. Measles in the skin, measles in the other lymphnodes, measles in the kidneys, measles in the gastrointestinal tract, even measles in the liver; that’s in part because the virus can hijack the cellular replication machinery in lining of the blood vessels and epithelial cells, as well as several types infection-fighting cells. That’s right: Measles replicates inside the very cells tasked with its destruction.

It’s macabre. That the virus is already jumping off to other people before the infected feels sick or notices a rash is one thing. That you can seem to have recovered from measles, only to die from it later, is quite another.

There are several ways measles can affect the brain and immune system on a longer timescale beyond a few sick days. For one, patients who clear the infection are left temporarily immunocompromised and open to new infections. “The link [between measles and post-infection immune suppression] is clear,” Dr. Diane Griffin explains via email. She’s the chair of the Department of Molecular Microbology and Immunology at Johns Hopkins School of Public Health, where researchers explore this and other phenomena.

However, it’s in the brain that measles really gets on the long game. In individuals who are immunocompromised before their altercation with measles, the virus can infect the brain, causing a fatal disease that progresses over the course of one to 12 months. This disease is called measles inclusion body encephalitis. Rarely, in some children and young adults, measles can cause a slowly progressive disease called subacute sclerosing panencephalitis. It’s the result of a persistent infection that often ends fatally 7 to 10 years later. “In addition, it can trigger an autoimmune demyelinating disease, post-infectious encephalomyelitis, soon after infection, that is frequently associated with neurologic deficits in those that survive,” Griffin says. In plain English? This means that the measles can turn your body’s militia against itself in such a way that it literally messes with your brain.

Thank goodness for vaccines!

Until the mid-20th century, humans were at the mercy of measles. Pretty much every kid got it, making it a common childhood rite of passage, albeit a lingering and occasionally deadly one. If you were young prior to John Enders and Thomas Peebles discovering the measles vaccine in the 1950s, chances are you spent some of childhood covered in red splotches. But with the advent of vaccination programs, humanity has been cracking down on measles — with some exceptions.

Ah, yes. The antivaccination movement. As public health agencies around the world are learning, perception is everything, and people have a hard time remembering dangers they cannot see. By successfully eradicating measles from society, organizations like the CDC and WHO created another problem entirely: compliance. Big Pharma makes a much better villain than the invisible monsters your grandma is always yabbering about, and really, who dies from a rash? I mean, 145,700 people a year — and that’s after the 75 percent drop that happened between 2000 and 2013 due to increased vaccination — but who’s counting?

But now, with parents opting out of childhood vaccinations— especially rich parents who can circumvent the public health system— measles is regaining its status. In California, where the latest outbreak started, there is a clear gap between the choices of the poorest and the most well-off.

What that means for the younger generations remains to be seen, but the future looks grim. We know that immunity gained from exposure to wild measles lasts at least 65 years, but what about the vaccine? “We do not know how long protective immunity lasts after the vaccine,” says Dr. Griffin. We know that 99 percent of recipients of the measles, mumps, and rubella vaccine can be protected for more than 20 years after two doses of the jab, but beyond that? I suppose we’re about to find out.

You can’t really blame us for forgetting, though: It’s an all too human reaction. After all, the saying “time heals all wounds” is just a nicer version of “don’t worry, you’ll forget how much this sucked.” The troubles ahead are what happens when we forget the real danger and focus only on what we perceive to be the biggest threat. Disengaging from the vaccine system becomes a nice, passive way to feel like you’re protecting yourself from “maybe” dangers. Suddenly, not vaccinating starts to look like caution. We’ve spent the last hundred years or so running around like public health mercenaries, isolating, defeating, and chaining up the monsters of old, but it seems that perhaps we’ve done the job too well. Would you wear a seat belt if car accidents stopped happening in the 1950s?