As the tally of people infected with a strain of measles traced to an amusement park in California continues to climb, health officials are warning that those who choose not to vaccinate themselves or their children will almost certainly be infected if they are exposed to the disease.
They also warn that the outbreak has not yet been contained, and some health officials are expecting the number of cases to grow.
Nearly 90 people spread across seven states and Mexico have been reported infected from an in Anaheim, California.
California health officials have confirmed at least 74 cases of the disease as of Tuesday, and health officials in Alameda County near San Francisco have placed 30 babies under home isolation.
Arizona Department of Health Services Director Will Humble toldThe Arizona Republic that the outbreak is at "a critical point," and that he expects the number of cases in his state will "absolutely" increase. He said as many as 195 children were exposed to a woman who had the disease last week.
The overwhelming majority of people infected are unvaccinated, including at least one infant too young to receive the immunization program.
The outbreak is especially striking, given the fact that as recently as 2000, health authorities had thought the existence of a highly effective vaccine made measles a problem of the past.
But the disease has resurfaced—there has been a median number of 60 cases a year since 2001 in the U.S. There were 220 cases in 2011, and there was an astronomical spike in 2014 of 640 cases, according to the CDC.
People infected with the disease still enter the United States—usually about 40 cases per year, according to Dr. Jane Seward of the Centers for Disease Control and Prevention. But widespread immunity through vaccinations had created a "wall of immunity" that typically stopped the disease from spreading whenever it came into the country from elsewhere.
As with the current outbreak, most spikes in cases occur among people who are unvaccinated—and who tend to cluster in geographic areas and often intentionally choose not to vaccinate themselves or their children. For example, half of the cases in 2014 were traced to Amish communities in Ohio. Victims there had traveled to the Philippines to work in relief efforts after a typhoon struck the country. Amish are not universally opposed to vaccinations, but tend to have lower immunization coverage.
"We have pockets of California and Oregon where as much as 10 to 20 percent of the population is unvaccinated," said Dr. Arthur Reingold, head of epidemiology at the University of California, Berkeley.
Many of the unvaccinated choose to abstain out of personal beliefs. Sometimes these are religious, but in many cases they have more to do with a fear of the negative risks of vaccines. Parents understandably want what they think is best for their kids, but such fears are "greatly exaggerated," Reingold said.
"We have enormous amounts of information and studies, on literally tens of millions of children, showing that any negative effects from vaccines are exceedingly rare and tend to be extremely minor," Reingold said. "I am alarmed that parents are getting more concerned about the risks of vaccines than they are about the risks of the diseases they prevent."
There is no direct cure or treatment for measles once a person has it. Measles is a virus, and unlike the antibiotics that treat bacterial infections, anti-viral drugs have not improved much since the 1950s, Seward told CNBC. Doctors can only manage the disease's symptoms and prevent complications. The best "treatment" is not contracting it in the first place.
But measles is an extremely infectious disease, much more so, for example, than Ebola—which, warranted or not, caused a scare among many people in the United States last year.
"Measles is really infectious, so if somebody isn't protected, they will almost definitely get it" if exposed to someone who has the disease, said Seward. "If somebody [with measles] coughs near them or even is in the same room, chances are greater than 9 out of 10 that they will come down with measles."
Measles still infects roughly 20 million people worldwide, and complications associated with the disease can include pneumonia, brain infections (which can lead to brain damage), bleeding, sepsis and other problems. Between 1 and 3 children out of every 1,000 will die from the disease. About 1 in 10 can suffer an ear infection that can lead to permanent hearing loss, and 1 in 20 will contract pneumonia, according to the CDC.
In some cases, measles can remain suspended in the air and infect someone even after an infected patient has left a room, Seward said.
Children too young to vaccinate are particularly vulnerable to the disease. A 2008 outbreak in San Diego was traced to an unvaccinated 7-year-old who had contracted the disease on a family trip to Switzerland and infected other children in pediatric offices before being diagnosed. One child infected was a baby who had to be hospitalized with a temperature of 106 degrees Fahrenheit.
"The baby's parents were worried the child was going to die," Seward said.
In another case, a baby returning from the Philippines was admitted to a child care center, and infected every other child in the room.
Adults are also vulnerable. Some were not vaccinated at all as children—either out of choice, or because they were young when the measles vaccination program was just beginning. Others may only be partially vaccinated—the measles vaccine (called MMR, for "measles, mumps, rubella") requires two injections for the best protection.
Often adults may not even know they are unvaccinated, said Seward.
"I have four adult children," Seward said. "I don't think they carry their vaccination records around. I can send a PDF to them, but not many parents can."