Dr. Harry Hull, the state epidemiologist, said the cases do not pose a threat to the general public because most people have been vaccinated against polio and are unlikely to have contact with Amish people. But he said he expects to find more infections within the Amish community because some of its members refuse immunizations on religious grounds.

None of the children have shown any symptoms of the paralyzing disease. About one in 200 people who contract the polio virus suffer paralysis because of it; others typically rid themselves the virus after weeks or months.

None of the four children had been vaccinated. Three are siblings; the fourth is a baby from another family.

Traced to an Oral Vaccine

The infection came to light when the baby was hospitalized for various health problems and underwent tests. Authorities then began testing other members of the community for the virus.

Officials would not identify the Amish community but said it consisted of 100 to 200 people.

Hull said the infections were traced to an oral vaccine that was administered in another country, probably within the past three years.

The use of oral polio vaccine containing the live virus was stopped in the United States in 2000. The live-virus vaccine caused an average of eight cases of polio a year in the United States. The US and Canada now use an injected vaccine made from the killed virus.

State and federal officials are investigating how an infection from a vaccine given in another country reached Minnesota. Stool or saliva from an infected person can transmit the virus.

Health officials said they are working with the Amish community to determine who may have been exposed to the virus, and to encourage immunizations.

Community Cooperation

"We have been going house to house, talking with them about the risk, offering the vaccine and attempting to collect specimens to see if the virus has been spreading," Hull said. "Some families have said, 'No, thank you, we do not want to interact with you at all.' Other families have said, 'Sure, we'll get vaccinated. We'll provide specimens.'"

Without the community's cooperation, Hull said, there is a chance of an outbreak similar to one that occurred in 1979 in Amish communities in Iowa, Wisconsin, Missouri and Pennsylvania. Ten people were left paralyzed by the disease, according to the Centers for Disease Control and Prevention.

The last naturally occurring case of polio in the United States was in 1979, and health officials consider the disease eliminated in the Western Hemisphere. It persists in other parts of the world, with the vast majority of cases concentrated in India, Nigeria and Pakistan, according to the World Health Organization.

According to the CDC, more than 95 percent of US children are vaccinated against polio by the time they enter school.