"Bingo," said Dr. Norman Crouch, the laboratory's director. "It was a 98 percent match. We knew we had nailed it."

The Minnesota laboratory sent the sample to the Centers for Disease Control and Prevention in Atlanta, which confirmed the results. Officials were immediately concerned about where the virus originated and where it might have spread.

Confirming the presence of polio in a city with even one infected person is not impossible, said Dr. Mark D. Sobsey, a professor of environmental microbiology at the University of North Carolina. The stool of an infected person contains so many viral particles that tests at a sewage treatment plant can reveal it. Such tests helped track outbreaks in the Gaza Strip and Haiti in recent years.

Since many Amish use outhouses, however, state officials geared up to go door to door. They unearthed a public health form explaining how to collect stool samples. The form had pictures of a flush toilet and a garbage can with a plastic liner -- things foreign to many Amish communities. Officials changed the form.

Gary Wax, an epidemiologist for the Minnesota Department of Health, contacted the leader of the Amish community where the child lives and asked for his permission to seek stool samples from those in his community. The leader gave his blessing, Mr. Wax said.

"We really tried to do it in a respectful way rather than just barge right in there," Mr. Wax said.

Since the Amish have no phones, he could not call for appointments. He and his colleagues knocked on doors. They had been warned against speaking directly to Amish women without their husbands present, Mr. Wax said, and the men were "running all over the place, helping each other with harvesting and construction." So if the man was not at home, they left.

"We came back many times to some places," Mr. Wax said. After weeks of effort, just 5 of 24 families in the community agreed to cooperate. Three of the five, including the family of the 8-month-old, proved to have infected children.