The World Health Organization said polio is spreading in Angola and has crossed into neighboring Democratic Republic of Congo, creating an international threat.
A vaccine for polio was discovered more than 50 years ago, but according to data released by the health group on Wednesday, the disease is gaining a foothold in central Africa.
In Angola, an outbreak that began in 2007 has this year spread to re-infect areas that had been cleared of the disease. It also has spread into a bordering province of the DRC.
Oliver Rosenbauer - a spokesperson for the organization's Polio Eradication Initiative - said, "At the moment it's the only expanding outbreak of polio in Africa so to us it's very concerning."
The situation in Angola and DRC comes as a disappointment, Rosenbauer said, in light of the progress made elsewhere in Africa.
Nigeria has seen a 99 percent reduction in polio cases this year, compared to the same period in 2009.
There have been no cases in West Africa since May and in the Horn of Africa it's been more than 12 months since a case has emerged.
Rosenbauer stressed that vaccination has to be comprehensive.
"They've done a number of immunization campaigns in Angola which, in most countries, these immunizations campaigns rapidly stop outbreaks. But they're not reaching sufficient number of children with their immunization activities," he said.
Rosenbauer said polio in one place creates a global threat and children are most at risk. Polio is a highly contagious disease that invades the nervous system and can cause complete paralysis within hours.
"It seems to be able to find any unimmunized child. And remember, polio is a very dangerous disease that - there's no cure for it - once you have it, you have it for life so the only means to protect yourself is by vaccination," said Rosenbauer.
Rosenbauer said an immunization campaign will take place in Angola and DRC this month.