Media playback is unsupported on your device Media caption BBC News outlines the struggle against polio - in 60 seconds

The World Health Organization (WHO) has declared the spread of polio is an international public health emergency.

Outbreaks in Asia, Africa and Middle East are an "extraordinary event" needing a co-ordinated "international response", the agency says.

It recommends citizens of affected countries travelling abroad carry a vaccination certificate.

It says Pakistan, Cameroon, and Syria "pose the greatest risk of further wild poliovirus exportations in 2014."

The WHO recorded 417 cases of polio worldwide for the whole of 2013. For 2014, it had already recorded 68 cases by 30 April - up from 24 in the same period last year.

Polio mainly affects children under five years old.

The virus is transmitted through contaminated food and water, and multiplies in the intestine. It can then invade the nervous system, causing paralysis in one in every 200 infections. It is capable of causing death within hours.

'Ongoing risk'

"The conditions for a public health emergency of international concern have been met," said Bruce Aylward, WHO Assistant Director General.

Image copyright bbc Image caption Poliomyelitis has existed as long as human society, but became a major public health issue in late Victorian times with major epidemics in Europe and the United States. The disease, which causes spinal and respiratory paralysis, can kill and remains incurable but vaccines have assisted in its almost total eradication today. Image copyright Getty Images Image caption This Egyptian stele (an upright stone carving) dating from 1403-1365BC shows a priest with a walking stick and foot, deformities characteristic of polio. The disease was given its first clinical description in 1789 by the British physician Michael Underwood, and recognised as a condition by Jakob Heine in 1840. The first modern epidemics were fuelled by the growth of cities after the industrial revolution. Image copyright Other Image caption In 1916, New York experienced the first large epidemic, with more than 9,000 cases and 2,343 deaths. The 1916 toll nationwide was 27,000 cases and 6,000 deaths. Children were particularly affected; the image shows child patients suffering from eye paralysis. Major outbreaks became more frequent during the century: in 1952, the US saw a record 57,628 cases. Image copyright Getty Images Image caption In 1928, Philip Drinker and Louie Shaw developed the "iron lung" to save the lives of those left paralysed by polio and unable to breathe. Most patients would spend around two weeks in the device, but those left permanently paralysed faced a lifetime of confinement. By 1939, around 1,000 were in use in the US. Today, the iron lung is all but gone, made redundant by vaccinations and modern mechanical ventilators. Image copyright Getty Images Image caption A major breakthrough came in 1952 when Dr Jonas Salk (L) began to develop the first effective vaccine against polio. Mass public vaccination programmes followed and had an immediate effect; in the US alone cases fell from 35,000 in 1953 to 5,300 in 1957. In 1961, Albert Sabin (R) pioneered the more easily administered oral polio vaccine (OPV). Image copyright Getty Images Image caption Despite the availability of vaccines polio remained a threat, with 707 acute cases and 79 deaths in the UK as late as 1961. In 1962, Britain switched to Sabin's OPV vaccine, in line with most countries in the developed world. There have been no domestically acquired cases of the disease in the UK since 1982. Image copyright bbc Image caption By 1988, polio had disappeared from the US, UK, Australia and much of Europe but remained prevalent in more than 125 countries. The same year, the World Health Assembly adopted a resolution to eradicate the disease completely by the year 2000. Image copyright bbc Image caption The WHO Americas region was certified polio free in 1994, with the last wild case recorded in the Western Pacific region (which includes China) in 1997. A further landmark came in 2002, when the WHO certified the European region polio-free. Image caption In 2012, Polio remains officially endemic in three countries - Afghanistan, Nigeria and Pakistan. Despite so much progress, polio remains a risk with virus from Pakistan re-infecting China in 2011, which had been polio free for more than a decade. previous slide next slide

He was speaking after last week's emergency meeting in Geneva on the spread of polio which included representatives of the affected countries.

"The international spread of polio to date in 2014 constitutes an 'extraordinary event' and a public health risk to other states for which a co-ordinated international response is essential," the WHO's International Health Regulations Emergency Committee said in statement.

Polio infected countries Afghanistan

Cameroon

Equatorial Guinea

Ethiopia

Iraq

Israel

Nigeria

Pakistan

Somalia

Syria

Source: WHO

"If unchecked, this situation could result in failure to eradicate globally one of the world's most serious vaccine preventable diseases."

The WHO also lists Afghanistan, Equatorial Guinea, Ethiopia, Iraq, Israel, Somalia and Nigeria as "posing an ongoing risk for new wild poliovirus exportations in 2014."

It is only the second time in the WHO's history it has made such a declaration, the first being during the swine flu pandemic of 2009, the BBC's Imogen Foulkes in Geneva reports.

The polio virus is endemic in just three countries - Pakistan, Afghanistan, and Nigeria. But attacks on vaccination campaigns in Pakistan in particular have allowed the virus to spread across borders.

Syria, which was polio-free for 14 years, was re-infected with the virus from Pakistan.

Refugees are still pouring out of Syria, to Jordan, Lebanon and Turkey, and checking whether all of them have been vaccinated will be impossible, our correspondent says.