Measles, a common childhood disease 50 years ago, has become a rare diagnosis since 2000 due to the success of the measles, mumps and rubella vaccine.

But state and Allegheny County health departments and Children's Hospital called an emergency news conference yesterday to announce that two preschool-age siblings from Westmoreland County have been diagnosed with measles, or rubeola.

The state health department also said later that the children's 33-year-old father, who received only one of two necessary MMR vaccinations as a child, also is infected.

Health officials said they acted quickly "to prevent further spread" of the disease, and continue trying to locate those people exposed to the family.

"We saw the children [at Children's] Saturday evening, and today we confirmed the diagnosis," said Dr. Marian Michaels, professor of pediatrics in the Division of Pediatric Infectious Diseases at Children's Hospital. "We worry it is the tip of the iceberg, and other families and people are incubating the virus."

Health officials declined to identify the family or where they live to preserve their confidentiality. Besides, they said, the only way to prevent infection is for anyone born since 1957 to receive both doses of vaccine.

The state Department of Health said the "probable source of infection" is the Children's Hospital emergency room, where the children were March 10 and 11 for an unspecified reason not involving measles. Department spokeswoman Stacy Kreideman said the state now has focused its investigation on pinpointing who infected the family.

But county health department spokesman Guillermo Cole said the location and human source of infection remain unknown. "The suspected exposure had to be someone, somewhere, sometime. The question is who, where and when?"

Dr. Michaels said the hospital is a possible source of exposure, but not the only medical facility the children visited earlier in March. Calling the state's conclusion premature, she said the hospital is checking every child who came into contact with the children during their earlier visit to the emergency room.

Measles, an acute, highly communicable rash illness, is caused by a virus transmitted by direct contact with infectious droplets or, less commonly, through the air. Incubation of measles, from exposure to the onset of rash, is seven to 18 days. The person remains contagious for the four days before and after the onset of rash, which begins at the top of the head then spreads downward to the body. The person also can experience a fever up to 104 degrees, a cough, runny nose and conjunctivitis.

While most people suffer no complications, 1 to 6 percent can get pneumonia and 1 in up to 2,000 people can get encephalitis, frequently resulting in permanent brain damage. Although death is a rare occurrence, children younger than 5 and adults older than 20 face a higher risk.

People potentially infected with measles should avoid public transportation, including commercial airlines and crowded indoor areas, a county fact sheet states. Those who suspect they have measles should call ahead before visiting a clinic or hospital so arrangements can be made for the health care provider to see the patient without exposing others to the measles.

The three new cases bring the state total number of measles cases to only nine since 2006 and 18 since 2003. State statistics do not include Philadelphia.

Dr. Michaels said children should be vaccinated in the first year of life with a second vaccination before entering school. The state requires children to be vaccinated before starting school, but medical and religious exceptions are honored. The state provides the vaccine for free for those without health insurance or unable to pay.

Dr. Bruce Dixon, executive director of the county health department, said about 85 percent of county students have been vaccinated, leaving a notable percentage who are not immune to measles.