Anti-vaccine groups have helped fuel Minnesota’s largest outbreak of measles in nearly 30 years, with 58 confirmed cases since the outbreak was identified in April.

Most of the cases are occurring among unvaccinated Somali-American children in Minneapolis, whose parents have been the targets of anti-vaccine propagandists, according to the state health department.

One of the most noteworthy fearmongerers is Andrew Wakefield, the discredited doctor who introduced the bogus idea that vaccines and autism are linked in a paper in a 1998 The Lancet study. (Since then, the paper has been retracted, Wakefield has lost his medical license, and studies in thousands of children have shown the vaccine is safe.)

Along with other vaccine deniers, Wakefield has continued to spread fears about the measles-mumps-rubella vaccine through public lectures and outreach campaigns in Minnesota over the past several years, according to reports in the Star Tribune. He also defended his fearmongering in the Washington Post by saying he’s just giving concerned parents information they want.

Because measles is one of the most infectious diseases known to man, health officials are bracing for more cases. Here are seven facts to know about the virus.

1) Measles was eliminated in the US in 2000 — but outbreaks linked to vaccine refusal have been popping up

If you were born before 1960, you might have come down with a measles infection. In the US, before a vaccine was introduced in 1963, there were 4 million measles cases with 48,000 hospitalizations and 500 deaths every year. That's just a massive burden of illness.

By 2000, because of widespread vaccination, the virus was declared eliminated in the United States: Enough people were immunized that outbreaks were uncommon, and deaths from measles were scarcely heard of.

Since then, however, public health officials have documented several measles outbreaks — a trend that that seems to be tied to some parents opting out of vaccines for their kids because of safety fears.

In 2014, US health officials recorded more than 660 cases of measles — the largest outbreak of the past decade — many of them linked to a community of Amish people in Ohio who were refusing vaccines.

In 2015, an outbreak originating at Disneyland in California helped spark nearly 150 cases — another widespread outbreak linked to vaccine denialism.

Now, there’s the outbreak in Minnesota. About 10 years ago, fears among Somali Americans that autism seemed to be disproportionately striking their children led to a University of Minnesota project on autism prevalence in the community. The researchers found that kids of Somali descent weren’t more likely than white children to have the disorder — but the concerns about autism were enough to open the door to anti-vaccine views.

Since 2008, the vaccination rate among Somali-American children has plummeted while remaining stable among non-Somali kids. Health officials haven’t yet identified patient zero in the current outbreak, though measles outbreaks in the US often originate with an infected traveler who brings the virus to an unvaccinated community.

Meanwhile, Somalia has been stricken with its own measles outbreaks and low rates of vaccination — but these trends are being driven by a lack of vaccine access because of famine and drought. There is some irony in the fact that immigrants and their families in the US are being persuaded not to take advantage of life-saving immunizations that their relatives back home may not be able to access.

2) There’s really no link between the MMR vaccine and autism

Large-scale studies involving thousands of participants in several countries have failed to establish a link between the MMR vaccine and the mental developmental disorder.

A study in the journal JAMA looked at nearly 100,000 children who got the shot and their family histories of autism. The researchers found the MMR vaccine was not associated with an increased risk of autism, even with children who had older siblings with the disorder. "These findings indicate no harmful association between MMR vaccine receipt and ASD even among children already at higher risk for ASD," the researchers concluded.

In another of the most thorough studies to date, nearly half a million kids who got the vaccine were compared with some 100,000 who didn't, and there were no differences in the autism rates between the two groups. "This study provides strong evidence against the hypothesis that MMR vaccination causes autism," the authors wrote in the New England Journal of Medicine.

Studies published in The Lancet, The Journal of Pediatric Infectious Diseases, PLOS One, and The Journal of Autism and Developmental Disorders, among others, have also found no association between the vaccine and autism.

3) Measles is a deadly, infectious disease that typically strikes children

After an incubation period of 10 to 12 days, measles comes on as a fever, cough, stuffy nose, and bloodshot and watery eyes. Loss of appetite and malaise are common, too. Several days after these initial symptoms, an uncomfortable spotty, rash begins to spread all over the body, starting on the face and neck, and moving downward. The rash usually lasts for three to five days and then fades away.

In uncomplicated cases, people who get measles start to recover as soon as the rash appears and feel back to normal in about two to three weeks.

But up to 40 percent of patients have complications from the virus. These usually occur in the very young (children under five), in adults over 20, and in anybody else who is undernourished or otherwise immunocompromised. Children under five have the highest probability of death.

The most common complication from the measles is pneumonia, which accounts for most measles-related deaths. Less frequently, measles can lead to blindness, croup, mouth ulcers, ear infections, or severe diarrhea. Some children develop encephalitis (swelling of the brain), which can lead to convulsions, loss of hearing, and mental retardation. Again, these complications mostly arise in people whose immune systems are already weakened because of their age, preexisting diseases, or malnutrition.

According to the CDC, the horrible mathematics of measles looks like this: One out of every 20 children with measles gets pneumonia; one in 1,000 will develop encephalitis (swelling of the brain); one or two in 1,000 children will die.

4) This is what measles looks like — and there’s no treatment

Doctors can help patients avoid the more severe complications (blindness, pneumonia) by making sure patients have good nutrition and enough fluids.

For eye and ear infections that can arise, doctors can prescribe antibiotics. And because measles depletes its victims' vitamin A levels, doctors usually give patients two doses of vitamin A supplements.

5) Measles is entirely preventable

Measles is prevented through the combination MMR (measles, mumps, and rubella) shot. The CDC recommends that children get two doses:

The first dose, at 12 to 15 months of age.

The second dose, at 4 to 6 years of age.

Immunity from the vaccine lasts for decades, but you should ask your health provider about booster shots if you’re an adult.

The vaccine is known to be extremely safe and very effective: it contains a live but weakened version of the virus, and it causes your immune system to produce antibodies against the virus. Should you be exposed to actual measles, those antibodies will then fire up to protect against the disease.

Again, side effects are rare and mostly very mild. According to the CDC, for example, fevers after the MMR vaccine occur in one out of six people, and mild rashes, in one in 20. More severe problems are virtually nonexistent: serious allergic reactions happen in fewer than one in a million cases. Deafness, long-term seizures, and permanent brain damage are "so rare that it is hard to tell whether they are caused by the vaccine." So the benefits of the vaccine — the protection of children and the communities they live in — vastly outweigh the harms.

6) One case of measles can lead to 18 others

If you’re not vaccinated, it’s extremely easy to get measles. In an unimmunized population, one person with measles can infect 12 to 18 others. That's way higher than other scary viruses like Ebola, HIV or SARS. (With Ebola, one case usually leads to two others. With HIV and SARS, one case leads to another four.)

Measles is an airborne virus, transmitted by respiratory droplets from the nose, mouth, or throat of an infected person, it’s usually shared through coughing or sneezing.

Small particle aerosols from someone with measles can stay suspended in the air for long periods of time after they’ve left a room, and the virus can live on surfaces for up to two hours. So a person with measles can cough in a room, leave, and hours later, someone else could catch the virus from the droplets in the air (unless they were vaccinated). No other virus can do that.

What worries health officials is that the measles virus can spread in a person four days before the onset of the telltale rash. So people with the virus start being contagious before they’d ever know they had measles. (They stop being contagious about four days after the rash appears.)

In exceptionally rare cases, even if you are vaccinated, you can still get measles. In fewer than five percent of vaccinated people, their immune systems just don't kick in even with the shots. Researchers aren't sure why.

Getting only one dose of the vaccine instead of the recommended two also seems to increase people's chances of getting measles if they're ever exposed.

7) Health officials are worried that vaccine refusal is on the rise

Let’s be clear about something up front: Most American children still get their shots. More than 90 percent of kids receive vaccines for polio, measles, mumps, rubella, hepatitis B, and chickenpox, though the coverage rates are slightly lower for other routine vaccines. Most American parents also say they support school-based vaccine requirements.

States started to mandate that school children get inoculated against diseases because we need vaccination rates to remain high to sustain what’s called “herd immunity.” For any vaccine to be effective and prevent outbreaks, a certain (high) percentage of people in a population need to be immunized. This keeps diseases from spreading easily and keeps vulnerable groups that can't be vaccinated protected (such as very young babies or people with allergies to vaccines).

And yet, since vaccination was invented more than 200 years ago, anti-vaxxers have been organizing. So how great is their influence? And is it growing stronger?

In the past couple of years, there haven’t been any new peer-reviewed studies on national trends in vaccine refusal. But the latest evidence we have from individual states, in combination with older studies on vaccine coverage rates and recent surveys of doctors, suggest there’s a growing problem in several parts of the country.

The main way to measure how many kids aren’t getting vaccinated by choice is finding out which proportion of kids get exempted from school vaccine requirements for nonmedical — that is, personal belief or religious — reasons. Since immunization laws are state-based, there’s variation across the country when it comes to the requirements.

As of August 2016, all 50 states have legislation requiring vaccines for students — but almost every state allows exemptions for people with religious beliefs against immunizations, and 18 states grant philosophical exemptions for those opposed to vaccines because of personal or moral beliefs (with the exception of Mississippi, California, and West Virginia, which have the strictest vaccine laws in the nation).

Leah Samuel of Stat News crunched the latest Centers for Disease Control and Prevention data on nonmedical exemptions from 2009 to 2016. (Her numbers weren’t peer-reviewed, however, like some of the other studies I’ll describe next.) She found that the volume of people seeking exemptions was greater in 2016 than at any other point since 2009 in 11 states: Connecticut, Florida, Iowa, Kentucky, Maryland, New York, North Carolina, North Dakota, Ohio, Oklahoma, and Virginia. She also found the national average for nonmedical exemptions was down in 2016 from a 2009 spike — a fact she attributes to California and Vermont’s 2015 cancellation of their personal belief exemptions.

Some of the best peer-reviewed evidence for an increase in the number of people refusing or delaying vaccines comes from Emory researcher Saad Omer. In one 2009 New England Journal of Medicine paper, Omer looked at the state-level rates of nonmedical exemptions. He found that between 1991 and 2004, those rates increased from less than 0.98 percent to about 1.5 percent. Again, this uptick was not spread evenly across the US, and even varied within states.

Generally, though, states that allowed only religious exemptions had a steady opt-out rate of about 1 percent during the period (1991 to 2004). But in states that were more lax — allowing philosophical or personal belief exemptions as well as religious and medical exemptions — the mean exemption rate increased from 0.99 to 2.54 percent.

In a 2012 follow-up to that paper, also published in NEJM, Omer found — once again — that “non-medical exemptions have continued to increase, and the rate of increase has accelerated.”

Doctors are also reporting that they’re meeting more and more parents who are skeptical of vaccines. The American Academy of Pediatrics periodically surveys its members and identified a rise in pediatricians reporting that they had patients who refused a vaccine — from 75 percent in 2006 to 87 percent in 2013.

As for state-level analyses, Baylor researcher Peter Hotez looked at the rate of nonmedical exemptions over the past 13 years in his home state of Texas. He found that in 2016, there were almost 45,000 children who refused vaccines — about double the number of exemptions in 2010 and a 19-fold increase compared with 2003.

Texas is one of those lax states that allow parents to get both religious and philosophical vaccine exemptions. And other studies have shown that people who live in places that make it easy to opt out of vaccines tend to have higher rates of exemptions. One paper found that states that allowed parents to refuse vaccines for philosophical or personal reasons had exemption rates that were 2.54 times as high as states that only permitted religious exemptions. Another older study, looking at data between 1991 and 2004, found an increase in exemption rates only in states that allowed philosophical exemptions.