Doctors in Yemen are trying to stop the spread of an outbreak of polio which has so far affected 22 people.
Children around the world are being vaccinated against polio
The country had been polio-free since 1996.
World Health Organization and Yemen Ministry of Health experts are currently investigating the source of the outbreak.
There are fears the number of people affected could rise, and doctors are instigating a vaccination campaign to try to stop the spread of the disease.
Experts say outbreaks can be quickly contained with high quality immunization campaigns which reach every child under five years old.
Four cases of polio were confirmed in Yemen on 20 April in the south-western part of the country, on the Red Sea coast.
The latest 18 cases occurred across the country, including in two districts in the country's capital Sanaa, suggesting the virus had spread across the country.
The WHO suggests that low immunisation rates among Yemen's children may facilitate the spread of the virus.
WHO representatives are now helping plan a vaccination campaign using the newest form of polio vaccine which protects against the type of the disease which is being seen in Yemen.
There are three types of polio which could be seen.
Yemen had already conducted a mass vaccination campaign because the country was considered to be at high-risk of polio re-infection from nearby Sudan where an outbreak of polio continues.
The number of polio cases around the world has been reduced from 350,000 in 1988 to 1,267 cases in 2004.
The disease remains endemic in six countries around the world.
But there is concern that the ongoing outbreak of polio in Africa might lead to re-infection of more countries in the polio-free Horn of Africa and the Middle East.
Yemen is the most recent of fifteen polio-free countries that have reported cases of polio since the epidemic began in late 2003.