Measles was nearly eliminated across the country nearly 15 years ago, an immunization victory over a highly contagious respiratory virus that once injured thousands and killed hundreds every year.
Most measles cases since then have been traced to people infected overseas. But a growing outbreak linked to Disneyland in Southern California — which spread among several unvaccinated children and adults — has touched a nerve, with health officials across the country warning about a rebound in diseases that had been rendered extremely rare.
In Baltimore, health officials raced this week to determine whether a 12-month-old girl had measles. The child was treated in emergency rooms at MedStar Good Samaritan Hospital and Johns Hopkins hospitals before samples were sent to be tested at the Centers for Disease Control and Prevention in Atlanta.
The case turned out not to be measles, but officials say it should serve as a reminder of the imperative to vaccinate.
Dr. Leana Wen, Baltimore's new health commissioner, criticized a movement by some parents who refuse to vaccinate their children, citing concerns of possible side effects. "It's a travesty that we'd eliminated measles and now it's back because some people aren't vaccinating their children," she said.
"Measles doesn't just cause death, it can cause permanent harm," including deafness and brain damage, she said.
Wen and other public health experts are concerned that in pockets with low immunization rates, preventable diseases such as whooping cough, polio and rubella could return after being stamped out, or nearly so, in the United States.
A growing number of people are wary of vaccines, producing "clusters of refusers around the country, like in California," said Karie Youngdahl, director of the History of Vaccines program in the College of Physicians of Philadelphia.
"Public health officials are seeing these as setbacks," she said.
But while outbreaks may become more common, especially where vaccination rates dip, Youngdahl said the nation won't return to a time before vaccines when the diseases were far more widespread. "We can take heart that they are not passing disease on like in the past," she said.
Earlier this month, Anne Arundel County's health department and school officials notified residents of an uptick in cases of pertussis, or whooping cough, mainly in school-age children. Officials said 11 cases were confirmed or suspected, and they asked the infected to stay home to stop the spread of the contagious respiratory infection. None of the patients were unvaccinated.
Maryland has had a few other outbreaks in recent years, including 87 people with mumps in 2013 and 369 with pertussis in 2012, state data shows. Only one or two cases of measles have been reported annually since the mid-1990s, according to Kurt Seetoo, an immunization epidemiologist with the Maryland Department of Health and Mental Hygiene.
Up to 5 percent of those vaccinated against measles have symptoms that mirror those of the disease, including fever and rash. That's what happened in the Baltimore case this week.
Measles is so contagious that it serves as a vaccine barometer — about 90 percent of unvaccinated people around someone with measles will catch it, and it can continue to spread hours after an infected person has left the room, according to the U.S. Centers for Disease Control and Prevention.
Experts say vaccination rates above about 95 percent may be the threshold needed to stop the continuous spread of the disease to those who can't be vaccinated, such as infants and children with leukemia and other conditions.
Maryland's rate for young children is just over 95 percent for the measles vaccine, which also includes mumps and rubella. The national average is about 92 percent, according to CDC data.
Other vaccinations have much lower rates. Maryland has a combined rate of about 75.8 percent for most vaccines, plus up to four boosters for each. The national rate is about 70.4 percent.
The required vaccines in Maryland for young and school-age children include measles, mumps, rubella, diphtheria, tetanus, pertussis, polio, chickenpox, hepatitis B and meningococcal meningitis.
"The state in general is doing well with vaccination," Seetoo said.
Jessica Dowdy, a Baltimore mother of five, said she's never seen any side effects and doesn't want her kids to experience any preventable diseases. She was recently getting her 5-year-old girl vaccinated at a city health clinic.
"There is so much stuff we have in the country we don't know about," she said. "My grandmother didn't think vaccines were necessary. … But they are necessary. You never know when a disease will come back and become a giant issue."
Measles once infected just about every child, up to 4 million annually, before the vaccine became available in 1963 and schools began requiring it and others.
CDC officials said this week that from 2001 to 2010, a median number of 60 measles cases were reported eachyear. But in 2014, there were 600 cases, the highest number in 20 years.
So far this year there have been more than 80 cases reported in 14 states. Many were linked to the Disneyland outbreak, where officials said many of those affected were not vaccinated and likely exposed to overseas travelers who likely didn't know they were contagious.
While there has been some level of resistance since the first vaccine for smallpox was developed in the late 1700s, more state legislatures began allowing more people to opt out of vaccines in the 1970s, Youngdahl said, some for religious reasons and others, like California, for philosophical objections.
The modern anti-vaccination movement also may have roots in the '70s feminist movement, when pregnant women began feeling the medical establishment was not looking out for their best interests, she said.
More recently, vaccine critics linked preservatives in vaccines to autism, though that theory has been debunked by many medical studies. Opposition now is more focused on ingredients believed to be unnatural or toxic and on the belief that too many vaccines are overloading infant immune systems or causing other harm.
Most who question vaccines, however, appear to pick and choose which vaccines to avoid. The CDC reports that only 1 percent to 2 percent of American children have gotten no vaccines.
The mainstream public health community strongly rejects claims of widespread harm from vaccines, citing a wealth of scientific study. Experts do acknowledge thatsome vaccines have been changed over time to make them more effective and to reduce side effects.
For example, the vaccine for pertussis was replaced when an average of one in 300,000 patients was experiencing serious side effects, said Dr. Alan Cross, a professor in the University of Maryland School of Medicine and associate director for adjuvant biology research in the school's Center for Vaccine Development.
(Adjuvants have been used for decades in some vaccines to improve the body's immune response but have become a target of some vaccine refusers who claim a link to autoimmune disease — which Cross calls a "remote concern.")
About the newer pertussis vaccine, Cross said it has fewer side effects but also is not as long-lasting. Researchers continue to investigate the best timing for boosters, including for adults.
"We're always trying to fine tune what has already been an effective strategy," he said.
He said the measles vaccine is still the most effective, working in 97 percent of cases with a booster. The mumps vaccine is less protective. And the influenza vaccine is never fully protective, even when officials match the vaccine against all flu strains circulating, which this year they didn't.
But he said the public is at great risk without vaccines, and those who don't want them may not have lived through a serious outbreak.
"If you've ever seen a child with pertussis, it's unforgettable," Cross said. "They whoop. You hold your breath and hope they take the next breath. And if you're exposed to someone with pertussis, or measles, and you're not immunized, you're highly likely to get it."