The affected physician come forward and identified herself in a video released Monday morning.
Dr. Jennifer McCabe, a physician at WellSpan Family Medicine -- Stony Brook, said she decided to disclose the information because she believes that public education about the virus is "in the best interest of our community."
First and foremost, she said in the video, it's vital that everyone who can be vaccinated with the MMR vaccine should do so.
"The vaccine is very safe, and very effective," she said. "Two doses of the MMR -- which is what the (Center for Disease Control) recommends, is 97 percent effective in preventing measles."
Some people are wondering if they should get a booster shot if they were vaccinated decades ago.
"We don't usually recommend that because that one dose is going to be 92-93% effective, and that's pretty good," Dr. Jessica Ericson, Infectious Disease Specialist at Penn State Health, said. "So if most people around you are also getting one or two doses of vaccine, it's going to be pretty hard for you to run into measles. On the other hand when we're more in an outbreak situation, it may not be a bad idea."
Dr. Erickson said people can get their first dose of the vaccine at 12 months old, and a second dose at age 4 or 5. Adults who haven't been vaccinated can get both doses one month apart.
But, McCabe said, there will always be a small percentage of people for whom the vaccine will not be effective.
"I am one of those people," she said in the video.
McCabe said she was vaccinated twice and twice tested positive for presumed immunity against measles three times in the last 20 years -- most recently within the past two days.
When the first measles case was confirmed in York County, McCabe said she received an immunization screening that confirmed she was presumed immune to the Measles.
"That is why I was able to continue seeing patients," she said.
WellSpan and McCabe followed all the CDC and Dept. of Health guidelines for Measles prevention, she said.
"Unfortunately, I still got the Measles," she said.
McCabe said her symptoms are not severe, crediting the MMR vaccine.
"We caught this very, very early," she said, noting she has not left her home since she began exhibiting symptoms, nor has she treated any patients.
"My vaccination did not prevent the Measles, but it did lessen the symptoms, and likely made me less contagious to the community," McCabe said.
WellSpan is working with the CDC and the Dept. of Health to notify anyone who may have been exposed as a precaution, McCabe said.