The outbreak of measles at Disneyland in Orange County, California, has reignited the debate over the anti-vaccination movement, driven by parents who question whether vaccines are safe and and whether there is a connection to autism in particular.
Medical experts say the study showing such a link has been repeatedly discredited and other parents counter their children are being endangered by irresponsible behavior.
Here’s what you should know.
How many people are affected?
One hundred and three people in California and and other states have reported contracting measles as a result of the outbreak that began at Disneyland in December, according to the national Centers for Disease Control and Prevention.
The majority of the children and adults who became ill either had not been inoculated or did not know if they had been, said Dr. Anne Schuchat, assistant surgeon general and director of the CDC’s National Center for Immunization and Respiratory Diseases.
“This is not a problem of the measles vaccine not working,” she told reporters this week. “This is a problem of the measles vaccine not being used.”
Since 2000, measles has been eliminated in the United States, meaning it is no longer native to the country. But it can still be spread by someone infected elsewhere and the CDC is assuming that is what happened at Disneyland.
How widespread is measles?
Each year there are 20 million cases around the world, and 145,000 people die, according to the CDC. Other complications: encephalitis and pneumonia.
Last year, there were a record number of measles in the United States, 644 cases, up from a median of 60 a year over the previous decade. And this year a total of 121 cases in 17 states and the District of Columbia have reported. The Disneyland outbreak represents 85 percent of the cases.
Those numbers pale compared to the average number of cases reported each year before the vaccine became available: 549,000.
Is there reason to worry?
The CDC's Schuchat said the numbers for January were concerning.
"I want to do everything possible to prevent measles from getting a foothold in the United States and becoming endemic again," she said.
Dr. Stephen Morse, professor of epidemiology at Columbia University's Mailman School of Public Health, said he thought the country was a long way from returning to the high number of measles cases and other diseases.
"If enough people are not taking these vaccines, we will see a resurgence, but right now these are fairly small events," he said. "So I think the reason everyone pays attention to it in medical and public health communities is simply because this is not a trend you would like to see really going up."
How high are vaccination rates?
Immunization rates remain high despite the attention the measles outbreak is receiving. Among kindergartners enrolled in the 2013-2014 school year, the median vaccination coverage was 93 percent and higher for measles, mumps, rubella, diphtheria, tetanus, whooping cough and chicken pox.
To provide what is called herd immunity -- to protect people who cannot be immunization and those for whom the vaccines are not effective -- experts recommend that between 90 and 95 percent of a community be fully inoculated. Health officials are worried about pockets of parents who are rejecting inoculation.
Morse said the control of a disease such as measles was hard won.
"When we actually had these diseases among us people feared them or at least really wanted a vaccine," he said. "Now of course we’re much more blasé, which is a mistake."
President Barack Obama weighs in
President Barack Obama, in a “Today” interview on Sunday, said parents had every reason to immunize their children.
"I understand that there are families that in some cases are concerned about the effect of vaccinations,” Obama said. “The science is, you know, pretty indisputable. We've looked at this again and again.”
What is the reaction from parents worried about vaccines?
Barbara Loe Fisher, the president of the National Vaccine Information Center, a Virginia-based nonprofit that advocates allowing parents to choose whether to vaccinate their children, said that it was premature to point fingers at those who decided to forgo vaccines.
"There is no question that there is a tremendous amount of pressure being placed on parents who are making informed vaccine decisions for their children," she said. "I think this has gone way too far. The discussion has gotten very ugly, it has gotten extremely polarized and it's caused a lot of parents to be very afraid of doctors and public health officials."
What about other diseases?
Mumps, rubella, pertussis or whooping cough and chickenpox are among others that could also spike if parents continue to forgo vaccinations, experts say.
“This isn’t just a measles problem,” said Dr. Gregory A. Poland, the director of the Mayo Clinic Vaccine Research Group in Rochester, Minnesota. “This is a problem for any transmissible disease for which we have safe and effective vaccines that aren’t unfortunately used.”
Measles is especially contagious, but there have been other outbreaks. Mumps, for example, is no longer common in the United States, with only 229 cases reported in 2012 compared to 186,000 cases each year before the mumps vaccination program began in 1967. But in 2009-2010, there were two large outbreaks, according to the CDC: one among mostly Hasidic Jewish children in New York who were delaying immunization, and another among mostly school aged children in Guam.