GENEVA (AP) – The U.N. health agency on Friday reported 18 new cases of polio in Yemen and said more people are believed infected, sparking fears of an epidemic in the Middle Eastern country with a low immunization rate among children.

The disease spread across the country from four cases that were recorded in the Red Sea port of Al-Hudaydah last week. Yemen previously was believed to be to be free of the disease.

“What we’re facing now is a major epidemic in Yemen,” said David Heymann, chief of the World Health Organization’s polio eradication campaign. The latest cases bring to 22 the number of confirmed instances of polio in the country.

Heymann said all the infected so far were children.

“Low immunization rates among Yemen’s children may facilitate the spread of the virus,” WHO said.

A nationwide immunization campaign is planned in May, Heymann said. WHO conducted one such vaccination drive last month, after it declared Yemen a high-risk country for polio reintroduction.

A third vaccination campaign might also be necessary in June, he added, expressing confidence “that this vaccine will help finish polio” in Yemen.

Yemen is the most recent of 15 previously polio-free countries that have reported new cases since 2003 after a vaccine boycott in Nigeria was blamed for causing an outbreak that spread the disease to other countries.

Hard-line Islamic clerics in northern Nigeria led the immunization boycott, claiming the polio vaccine was part of a U.S.-led plot to render Nigeria’s Muslims infertile or infect them with AIDS. Vaccination programs restarted in Nigeria in July 2004 after local officials ended the 11-month boycott.

Polio then re-emerged in Sudan, where it had been eradicated in 2001, and the strain was traced to Nigeria. The disease has infected 149 people in Sudan.

The Nigerian polio strain then spread to Saudi Arabia and Ethiopia, but vaccination campaigns averted major outbreaks in those countries.

The virus also spread to other African countries – Benin, Chad, Cameroon, Botswana, Burkina Faso, Central African Republic, Ghana, Guinea, Mali, and Togo.

WHO has yet to determine whether the strain in Yemen was the same as that which spread from Nigeria.

“It will never be possible to tell how this virus came into Yemen,” Heymann told reporters. “What’s important is that the virus is there and that we have to stop it.”

Polio is still endemic in Nigeria, India, Pakistan, Niger, Afghanistan and Egypt.

Last year, some 1,267 people were infected in the world – with 792 of those in Nigeria. The total new cases in 2005 stands at 110, according to WHO, with Nigeria, Sudan and Yemen listed as the worst-affected countries.

When WHO launched its anti-polio campaign in 1988, the worldwide case count was more than 350,000 annually.

Polio is a waterborne disease that usually infects young children, attacking the nervous system and causing paralysis, muscular atrophy, deformation and sometimes death.