India will make "extraordinary efforts" to immunize children against polio in its eastern state of Bihar, fearing disruption by massive flooding may make them more vulnerable to the crippling disease, officials said on Thursday.

The country of 1.1 billion people has the world's highest number of polio cases, with 139 being reported in 2007 out of a global total of 345. Defeating the paralyzing disease in India is seen as key in the global war against the virus.

In Bihar, where 20 cases of the virulent Type 1 polio were reported this year, nearly 15 million people -- including millions of children -- have been affected in what officials say are the worst monsoon floods in memory in some areas.

The WHO said with thousands of marooned or homeless children living in crowded makeshift camps in the state of 90 million people -- often in extremely unhygienic conditions -- the transmission of the virus could become easier.

"In flood-hit areas, populations mix in unusual fashion which facilitates transmission of the polio virus," said Hamid Jafari, head of WHO's polio project in India.

"There are operational challenges in reaching children in flood-hit areas and going house-to-house to vaccinate them," he said, adding that campaigns were under way in areas health workers could access.

Officials were planning to deploy boats in some areas where there was no let-up in flooding.

Polio is transmitted through the fecal-oral route in unhygienic conditions. It enters the intestine and multiplies there if food is eaten with unwashed hands.

Impoverished Bihar has the second highest number of polio cases after the most populous state of Uttar Pradesh. Most of Bihar's cases were reported from districts hit by flooding.

A spike in cases in 2006 in India -- mostly in these two states -- sparked global concern as the Indian strain of the virus reinfected several other countries.

India says it has beaten back the Type 1 virus in the western parts of Uttar Pradesh -- the world's most affected region -- for the first time, with no cases reported this year.

Officials said they did not want the progress in Uttar Pradesh undermined by gaps in Bihar due to the flooding.

"Diarrhea and intestinal infections that follow flooding compromises the efficacy of the polio vaccine," Jafari said. "We must make extraordinary efforts to plug the gap."