As measles outbreaks continue around the country, a large Northern California hospital is warning at least 200 patients that they may have been inadvertently exposed to the disease by a child who showed up in the emergency room with respiratory symptoms.

The 7-year-old girl was brought to the ER two weeks ago with “some symptoms of measles, but not the typical textbook characteristics of measles,” said Dr. Dean Blumberg, chief of pediatric infectious diseases at the University of California, Davis, Children’s Hospital. “Measles when it starts, starts off very similar to a lot of common community associated respiratory viruses.”

In some children, Blumberg said, the disease can start out with “fever, runny nose, runny eyes and cough. And before the characteristic rash develops, sometimes it can be very difficult to diagnose.”

“The patient came to the emergency department on March 17 and the patient was suspected of having measles on March 19,” Blumberg said. “At that time, the patient was placed in appropriate isolation.”

The UC Davis case underscores one of the difficulties of containing the disease: infected people can have it for days without knowing it and come in contact with many others, spreading it even before showing symptoms.

“Most cases of measles occur eight to 12 days after exposure, but it can occur up to three weeks after exposure,” Blumberg said. “Even vaccinated individuals can get measles because the vaccine is not 100 percent perfect. We need a very high vaccination rate because when measles is introduced to communities that have parents who don’t immunize their children, these clusters of cases can occur.”

Those clusters of cases could be the harbinger of worse things to come.

Back in 2000 measles was considered to be eliminated in the U.S., a major public health victory.

Before the vaccine became widely available, an estimated 400 to 500 people died each year, according to the Centers for Disease Control and Prevention. There were 48,000 hospitalized with the disease each year and 1,000 who suffered brain swelling, or encephalitis.

“We may be approaching the level where measles could be considered again to be endemic in the United States.”

As vaccine-hesitancy has grown, measles has begun to reappear. So far this year there have been at least 387 reported cases of measles in the U.S. across 15 states, according to the Centers for Disease Control and Prevention. That's compared to 372 reported cases of measles for all of 2018. Many of the cases in 2019 are clustered in six outbreaks, including Rockland County, New York and Clark County, Washington. This is largely due to a growing vaccine-hesitancy movement powered by misinformation campaigns.

According to the CDC, 91.1 percent of U.S. children between 19 months and 3 years old have received at least one dose of the mumps, measles and rubella (MMR) vaccine, well below the 95 percent needed to achieve herd immunity. In California, the percentage is slightly higher at 92.9 percent.

“With the drop in vaccination rates, we may be headed back in that direction,” said Roberto Cattaneo, a measles researcher and a professor of biochemistry and molecular biology at the Mayo Clinic in Rochester, Minnesota.

“It seems that we may be approaching the level where measles could be considered again to be endemic in the United States,” Cattaneo added.

One problem is that doctors may not recognize measles right away because they’ve never seen it, Cattaneo told NBC News. “I think doctors now will be alarmed and go back to their books and get accustomed to what it looks like.”

Parents who suspect that their children might have measles should call the hospital before arriving, so that they can adequately prepare, Blumberg cautioned.

“What we can do in those kinds of situations is let them in through the side door and place them immediately into an isolation room,” said Blumberg. “We don’t want them in the waiting room where potential exposures may occur.”