As predicted, the source of the measles outbreak in southwestern Pennsylvania was a foreign visitor, a child from India.

While neither the child nor any of five other people infected with measles have been identified, their whereabouts late in March included visits to Seven Springs Mountain Resort in Somerset-Fayette counties, Children's Hospital of Pittsburgh of UPMC, Alle-Kiski Medical Center in Natrona Heights and various trips on Allegheny County Port Authority buses.

The state Department of Health and Allegheny County Health Department said other "probable" cases of infection await confirmation.

But the source of infection was "a traveler from India" who arrived March 7 in the United States.

That the source was a child is confirmed by the investigation's focus at Children's Hospital, where two Westmoreland County preschool-age children contracted measles, which also infected their 33-year-old father.

An infected person, not necessarily the Indian child, was present at an Emergency Medical Services conference at Seven Springs between 9:30 a.m. and 3:30 p.m. March 26, where the person spent time in the Snowflake Room and main dining room.

At 6 p.m. March 27 through 3 a.m. March 28, an infected person was in Children's Hospital Emergency Department.

Between 10 a.m. and 3:30 p.m. the next morning, an infected person was at the Alle-Kiski Medical Center Emergency Department, including the main lobby and ground-floor public area. An infected person also spent time at Children's Hospital between 2:30 p.m. and 5 p.m. that same day, March 28.

Even before that, a person infected with measles spent many hours on local buses.

Between 11 a.m. and 1 p.m., March 24, an infected person rode the 71A bus, then again from 11 a.m. to 2 p.m. March 27.

Next, a person with measles rode the 33X West Busway for all stops between 10 a.m. and 1 p.m. March 27. The next day, March 28, an infected person took the Airport Flyer (28X) between 2:30 to 4:30 p.m.

Health officials said they no longer see any exposure risk at these locations.

But Dr. Ron Voorhees, the county health department's chief of the Office of Epidemiology and Biostatistics, said subsequent infections could extend the investigation for weeks to come.

"We're talking the end of May before we're confident that it will smolder out," Dr. Voorhees said. "One thing that's important in public health is to alert people in case they have not been diagnosed. We have contacted all the people we know how to reach, including children and other health facilities who we've been able to contact directly."

Although thousands have been contacted, he said, there is still the risk that others are infected or face infection. Health officials have advised some non-immune people who might have been exposed to infected people "to stay home until they are cleared of risk."

"For bus routes and waiting areas, we don't know who was there, so we want to get the word out so people potentially exposed can be alerted," Dr. Voorhees said.

Measles is extremely contagious. Dr. Voorhees noted a case in Greenland in which a person with measles infected 200 people attending a dance.

Measles involves fever, a rash beginning at the top of the head and spreading across the body, a runny nose and conjunctivitis. Only a small percentage suffer such side effects as encephalitis and pneumonia, with an occasional death among very young patients and those 20 and older.

But high vaccination rates in Allegheny County means most are immune, reducing the size of the outbreak, Dr. Voorhees said. Measles, a common childhood disease 50 years ago, is rare this century in the United States due to the successful measles, mumps and rubella vaccine. The state has had only about 20 cases since 2003. It was that fact that pointed to a foreign visitor as the source of the outbreak.

A list of other exposure sites is available at www.health.state.pa.us and www.achd.net. Anyone who has had a fever and rash any time since March 15 should contact a primary health care provider immediately.