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Three children - all members of the same family - have now been found to be infected with the virus. Health department disease investigators say there are direct links between the family of the three children and the family of an infant found to be infected with the virus at the end of September. The two families are not related by blood.

Like the infant, the three children do not have any symptoms of paralytic polio, and, like the infant, they had not been vaccinated against the illness.

Health officials are not providing any additional information about the cases because the information could be used to identify the children involved, in violation of the Data Practices Law.

Both of the affected families are members of the Amish community in central Minnesota. Health officials are working with the Amish community to identify people who may have been exposed to the virus - and make sure people who may be at risk have been immunized against polio.

The three new cases were identified through efforts by local, state, and federal public health officials to follow up with people who may have been exposed to the virus - either through direct contact with the infant or through contact with individuals who had contact with the infant and became infected.

"These cases do not represent a generalized outbreak of polio virus infection," said Minnesota Health Commissioner Dianne Mander-nach. "You are not at risk of infection unless you have had close contact with an infected person, and you have not been immunized against polio. The general public is not at risk."

An estimated 93 percent of Minnesota infants have been immunized against polio, and 98 percent of Minnesota children have been immunized by the time they start school, Mandernach said.

The health department has been focusing its disease investigation on the Amish community and on health care workers who may have provided care to the infected infant, according to Dr. Harry Hull, Minnesota State Epidemiologist.

"We will continue to follow up directly with people who may have been exposed to the virus," Dr. Hull said. "Unless you are contacted directly by a health care or public health worker, you are not at any risk of infection in connection with these cases."

"However, these cases do serve to underscore the importance of making sure your immunizations are up to date for polio and other vaccine-preventable diseases," added Dr. Hull.

Because members of the central Minnesota Amish community have links with Amish groups in other parts of the country, it's possible that cases of infection may eventually be identified outside Minnesota, Dr. Hull said.

Health department officials say the virus strain found in the four central Minnesota cases appears to be a variant of the strain used in oral polio vaccine, which is still used in some parts of the world, but not in the United States.

Use of the live-virus oral vaccine was discontinued in the U.S. in 2000. All polio vaccinations in the U.S. are now done with an injected, killed-virus vaccine.

Before use of the live-virus vaccine was discontinued, it caused about eight cases of paralytic polio a year in the U.S., on average. The last case of naturally occurring polio in the U.S. was reported in 1979. Naturally-occurring polio is considered to be eradicated in the western hemisphere.

Paralytic disease is relatively uncommon in people who are infected with the polio virus, occurring in only about one of every 200 people infected.