Georgia health officials said Monday that they have identified the likely source of a measles outbreak in Cobb County, linking a spate of illnesses to a local family of five that traveled to Florida in late September.

Members of the Cobb County family – all of whom were unvaccinated – likely became infected with measles during that trip to Florida, the Georgia Department of Public Health said. The illnesses were never reported to Georgia health officials, who only learned the family's diagnoses in the course of their investigation into the outbreak of the highly contagious disease.

Also on Monday, the state health agency announced two new cases of measles – siblings of a recently diagnosed Mabry Middle School student. They also were unvaccinated, but are not members of the family infected in September.

The total number of confirmed cases in Cobb County stands at 11, and the total for the year in Georgia at 18. At least 17 of those sickened with measles were unvaccinated.

Health officials said all the cases in Cobb County are limited to three families who live in the same vicinity with children who have spent time with each other. Since the Mabry student was diagnosed, those exposed, including unvaccinated students and at least one adult at the school, have been staying at home.

More and more parents are choosing not to vaccinate their children. Our kid reporter, Sammy, explains how this is putting unvaccinated children and vulnerable people who come in contact with them, like infants or grandparents, at greater risk.

If the number of cases doesn’t rise during an incubation period that ends Nov. 22, officials said they are hopeful that will signal that the outbreak is contained.

In the United States, most measles cases are the result of international travel. The virus is usually brought here by people who get infected in other countries. Then those travelers spread the disease to people who have not been vaccinated.

State health officials are asking anyone with symptoms of measles to call a health-care provider first before going into a doctor’s office or hospital.

The measles virus spreads when an infected person coughs or sneezes. Droplets from the nose or mouth become airborne or land on surfaces where germs can live for two hours.

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Measles is so contagious that, if one person has it, up to 90%of the people around him or her also will become infected if not vaccinated.

In Georgia, vaccinations are mandatory for public school attendance, but there are exemptions for medical and religious reasons.

An estimated 93.6% of young children in Georgia received the recommended vaccination for measles, mumps and rubella, slightly lower than the national average of 94.7%, according to research published in an October issue of the Centers for Disease Control and Prevention Morbidity and Mortality Weekly Report.

Also in Georgia, 2.5% of kindergartners had an exemption from at least one vaccine, which is the same overall percentage for the U.S.