Health experts warn there has been a global increase of people susceptible to polio because of not being exposed to the disease previously. Unvaccinated and absence of exposure to the virus makes adults most at risk.

Reports say that there are 19 confirmed and 150 suspected polio cases in Namibia. The country had been polio-free since 1995.

Polio is an infectious disease that attacks the nervous system and can lead to paralysis. It is spread through contaminated food or drinking water, contact with infected feces or contaminated swimming pool water.

While virtually non-existent in most countries, it is still endemic in some countries.

The Namibian outbreak infected people aged 15 to 45.

Bruce Aylward, head of the World Health Organization's polio eradication drive, told the BBC: "This just shows that we have to eradicate polio everywhere because, while endemic areas persist, the virus will find susceptible people."

He added: "There are bigger immunization gaps than at any time in the last 25 years.