International aid groups said Thursday they're launching a mass polio immunization campaign in three Central African nations in response to a polio outbreak suspected in more than 100 deaths and deemed "unusual" because it targets adults more than children.

The World Health Organization, UNICEF and Rotary International said they will begin vaccinating some 3 million people in the Republic of Congo, its larger neighbor Congo and nearby Angola on Friday. The outbreak of the highly infectious disease was confirmed on Nov. 4 in the Republic of Congo.

The emergency immunizations will begin in the coastal port city of Pointe Noire in Republic of Congo and will continue through the end of the year.

The agencies said in a statement Thursday they had counted 97 deaths out of 226 cases of acute paralysis that are suspected to be polio cases. They noted that of those, four have been confirmed as polio.

The groups also expressed concern that young adults between the ages of 15 and 29 were being affected. Health officials in the Republic of Congo said some patients are as old as 43. Polio, which can cause lifelong paralysis, is considered a childhood disease.

The impact on older patients, WHO's Africa director said, demonstrates that the population does not have full immunity.

"Every man, every woman, every child will be immunized irrespective of their past immunization status," said Dr. Luis G. Sambo, WHO's regional director for Africa. "This way we can be assured that everybody is reached, including young adults, whose immunity may be low."

Last month, the WHO launched a 15-country Africa campaign, beginning in Angola and the larger Congo, which together had seen more than 50 cases. Their volunteer campaign also focuses on West Africa, where Nigeria, the most populous nation in sub-Saharan Africa, has never managed to eradicate the disease. On the day that campaign was declared, officials in Uganda — which was most recently declared polio-free in 2009 — announced an outbreak of the disease in their country.

Polio is infectious and mainly strikes children under five. It is spread primarily by the feces of an infected person getting into the food chain. Besides causing paralysis, it can be fatal.

The disease has dropped by more than 99 percent since the WHO and partners launched an initiative to eradicate the disease in 1988 through vaccinations. But the numbers of cases — fewer than 2,000 annually — have remained at a virtual standstill since 2000. Polio persists in a handful of countries, including Afghanistan, India and Pakistan.