The polio virus has been found in sewage samples near Sao Paulo in Brazil, one of the host cities for the current World Cup football tournament, the World Health Organization said.
No human case of the disease has been reported in Brazil so far.
The polio virus found was found in sewage collected in March at Viracopos International Airport. It is a close match with a recent strain isolated in a case in Equatorial Guinea, the organisation said.
Several national football teams have arrived to Sao Paulo via the Viracopos airport.
The United Nations agency said that it assessed the risk of further international spread of polio virus from Brazil as "very low."
Brazil has been polio-free since 1989.