Tracking the source of measles that infected three members of a Westmoreland County family likely will lead to a foreigner or an unvaccinated individual who had foreign contact.

And while health officials said they're certain they'll track down everyone who came in contact with the family, they may never find the source.

"Ultimately, this came from a foreign country," said Dr. Jim Lando of the Allegheny County Health Department. "We usually find the cases [of exposure], but sometimes we don't find the source."

The county health department and the Pennsylvania Department of Health, with cooperation from Children's Hospital of UPMC of Pittsburgh and another medical facility where the infection could have occurred, are working to track down the source of measles that infected two preschool-age children and their 33-year-old father.

Health officials refuse to name the family due to confidentiality laws and also declined to say exactly where the family lives. Instead, Dr. Lando said, people who have not received the measles, mumps and rubella vaccination or whose children have not received both doses of the MMR vaccine could best prevent infection by receiving the full vaccine.

"It's very infectious, so we need to find those people who are susceptible and may be exposed so we can disrupt transmission," Dr. Lando said. "Vaccines have done an incredible job of preventing disease, and precisely because of that success, people fail to remember the diseases they are protected against and make decisions not to vaccinate -- and at a cost."

Dr. Lando said the investigation involves 20 officials from the county and state health departments, along with medical facilities where the infected family had been March 10 and 11, when they likely were exposed to the virus.

That effort goes in two directions, Dr. Lando said, using a navigation metaphor. Investigators are working upstream to find the source of infection. Then they'll work downstream to find everyone who had contact with that source. They also are trying to track down people who came in direct contact with the family.

People vulnerable to infection can be vaccinated. Those already incubating the disease could be given an antibody globulin to prevent full infection.

"We are tracking down every place the family went and the types of interaction," Dr. Lando said. "The fact they were traveling around casually is not as big a deal. That's not as concerning as the amount of time they spent ... breathing air with other people."

Measles is a highly communicable rash illness caused by a virus transmitted by direct contact with infectious droplets or, less commonly, by airborne spread. The incubation period is seven to 18 days, but a person is contagious for four days prior to the appearance of the rash and remains contagious four days after the rash appears.

Measles is rare in the United States due to laws requiring vaccination. Dr. Lando said up to 98 percent of the population has been vaccinated or is immune. Some people don't get vaccinated due to medical or religious exemptions.

But the World Health Organization estimates that more than 30 million people worldwide are affected each year by measles. It said about half of all imported measles cases occur in U.S. residents returning from visits abroad.

One in up to 2,000 cases can lead to encephalitis that can result in permanent brain damage. Death is rare but most often occurs in children under 5 and adults 20 or older.

"We keep drawing circles around each case until we find all the people who may be exposed and who are susceptible, and see if they need to be treated," he said.

The state is testing a sample taken from another person suspected of exposure and possible infection. That represents the only new lead in the case, Dr. Lando said.

"This is the bottom line: Everyone else can handle this by getting their kids vaccinated," he said. "That's what this is an example of -- the failure of people to adequately vaccinate their children."