The United States is in the middle of what may end up the largest measles outbreak in two decades.

Dozens of new cases are being reported each week. As of April 11 — date of the most recent data published by the U.S. Centers for Disease Control and Prevention — 555 cases had been confirmed.

That’s just 112 shy of the number of cases reported for all of 2014, the worst year on record since measles was eliminated in the U.S. in 2000. The current outbreak was triggered at multiple spots around the country, but it’s being fueled almost entirely by one source: large pockets of unvaccinated people.

In states like Washington and New York, public health authorities have been racing to immunize children in vulnerable communities, and bumping up against large clusters of parents who refuse to vaccinate.

But in California, it’s a different story. Though the state has seen a handful of small outbreaks this year, including seven Bay Area cases, none are nearly as dramatic as what’s happening in other parts of the country. And that, public health experts say, is largely due to the state tightening its vaccination requirements three years ago — after learning a hard lesson of its own.

Before the vaccine: half a million cases a year

Measles is a highly contagious disease caused by a virus. People generally show symptoms about a week or two after exposure, and will first have a high fever, cough, runny nose and red, watery eyes. A few days later, the characteristic red rash appears, usually starting on the face and spreading down the body.

Before the vaccine was approved and became widely available in the mid 1960s, measles was a common childhood illness, with half a million cases reported every year. Though most people recover without any complications, measles can be deadly; before the vaccine, about 500 people per year would die. About as many would suffer permanent injuries like hearing loss and brain damage.

After the vaccine, the number of measles cases fell sharply. By the 1980s, it had disappeared in most parts of the country. Measles was declared eliminated when the CDC reported that it was no longer circulating naturally in the U.S., meaning any new outbreaks were sparked by the virus being carried in from another country.

Since then, annual cases have ranged from a low of 37 to a high of 667. The last measles death was reported in 2015.

Post 2000: the anti-vaccination era

In 1998, a since-disgraced British doctor published a report suggesting the measles vaccine could cause autism. The report, which was a simple case study of a dozen children, was immediately criticized by many experts in autism and vaccines, and 12 years later it would be fully debunked, and retracted from the science journal that published it.

But by then, the damage had been done and a global anti-vaccination movement had taken off. Less than a decade after measles was eliminated in the United States, cases began creeping up again, largely due to expanding clusters of parents refusing to vaccinate, infectious disease experts said.

Misinformation and measles in California

The anti-vaccination movement got an especially strong foothold in California, where state law allowed parents to easily opt out of vaccinating their children. As vaccination misinformation spread among parents at schools, in playgroups and especially on social media, childhood immunization rates fell statewide — eventually below 92 percent, the level needed to prevent community spread of measles.

At the same time, measles cases picked up — slowly, at first. Then there was the Disneyland outbreak.

By late 2014, a decade of falling vaccination rates had left California vulnerable, with large pockets of unimmunized people up and down the state. A person with measles, most likely contracted somewhere in Asia, went to a hugely popular amusement park and exposed the virus to tens of thousands of people, who then went back to their own communities across the state.

Within the first two months of 2015, California had more measles cases than the previous 10 years combined.

Rise and fall of vaccination rates

The Disneyland outbreak prompted fierce public pushback against the anti-vaccination agenda. By summer 2015, the state Legislature had passed a new law doing away with rules that allowed parents to opt out of vaccinating their children for any reason other than strict medical exemptions.

And by the 2016 school year — the first after the new vaccine policy was implemented — vaccination rates statewide had climbed well over 92 percent.

This year, as other communities around the country are struggling to control massive waves of new measles cases, California has been reaping the rewards of its strict vaccination laws, public health experts say. New York state has had more than 300 cases so far this year. California: 21.

Erin Allday is a San Francisco Chronicle staff writer. Email: eallday@sfchronicle.com Twitter: @erinallday