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The infant, who had previously been diagnosed with immune system problems, does not have symptoms of the paralytic illness that can sometimes result from a polio infection and is currently hospitalized. Further information regarding the infant cannot be released due to state and federal data practice laws.

Health department officials emphasize that only people who have had direct contact with the infant - including unimmunized health care providers and family members - are at any risk of illness in connection with this case.

At this time, no additional cases of infection with the polio virus have been reported in connection with the infant. The general public is at no risk from this case.

This is the first case of polio infection reported in the U.S. since 2000, when use of live-virus oral polio vaccine was discontinued in the country. All polio vaccinations in the U.S. are now done with an injected, killed-virus vaccine.

Before 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 was reported in the U.S. in 1979. Naturally-occurring polio is considered to be eradicated in this hemisphere.

Health officials say the virus strain found in the Minnesota infant appears to be a variant of the strain used in the oral vaccine, which is still used in some parts of the world.

"The Minnesota Department of Health is working with local and national officials to ensure the safety of the general public and to investigate this case," said Minnesota Health Commissioner Dianne Mandernach. "It is important to note that there is no risk to the general public."

"However, the polio virus can cause a very serious form of illness, and it was a major public health concern before the first polio vaccines were introduced in the 1950s. We know many people still have vivid memories of that era, so we wanted to reassure everyone that this event does not signal a resurgence of polio."

"Only unvaccinated people who have had direct contact with the infant are at risk, and we are following up directly with them to make sure their immunizations are up to date," added Kris Ehresmann, who heads the Immunization, Tuberculosis, and Immigrant Health Section at the Minnesota Department of Health. "If you don't hear from a public health or health care official, you're not at risk."

Officials say it is unknown at this time how the infant became infected, but the virus is transmitted through direct contact with the stool or oral secretions of an infected person. Health department officials are working with the Center for Disease Control to investigate the case.

Although members of the general public are not at risk, health officials say this unusual case should serve as a reminder to make sure that all of your immunizations are current and that children receive immunizations as recommended.

"It's always a good idea to check with your physician or health care provider to be certain all of your vaccinations are current," Ehresmann said. "Make sure you're protected and make sure your children are protected."

An estimated 93 percent of Minnesota's population has had the full primary series of three polio shots, which are usually administered in infancy.