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The second of three forms of the polio virus has been eradicated, experts have announced.

There are three types of the wild polio virus, which, while scientifically different, cause the same symptoms, including paralysis or even death,

The world was declared free of type 2 four years ago - and now the World Health Organization has said type 3 has also been eradicated.

But type 1 is still circulating in Afghanistan and Pakistan.

Polio usually affects children under five. The WHO estimates one in 200 cases leads to irreversible paralysis. Death can occur when breathing muscles are affected by the paralysis.

There is no cure but the polio vaccine protects children for life.

Cases of wild polio have fallen by 99% since 1988.

The declaration type 2 had been wiped out was made in 2015, a full 16 years after the last case was seen in India.

It has been seven years since the last case of type 3 polio was detected, in northern Nigeria.

Since then, experts from the Global Commission for the Certification of Poliomyelitis Eradication, of which the WHO is a member, have watched patterns of polio cases to be sure type 3 had been eradicated.

Speaking on World Polio Day, Dr Matshidiso Moeti, the WHO's regional director for Africa, said: "The eradication of wild polio virus type 3 is a major milestone towards a polio-free world - but we cannot relax.

"Countries must strengthen routine immunisation to protect communities, ramp up routine surveillance so that we are able to detect even the slightest risk of polio re-emerging and ensure the timeliness and quality of outbreak response in the event that a case is detected."

Type 1 is now circulating in Afghanistan and Pakistan only. The last case detected case in Nigeria, where it was also endemic, was in 2016.

But Dr Moeti added: "This job is not finished until wild polio virus type 1 is globally eradicated, along with concerning outbreaks of circulating vaccine-derived polio virus."

Vaccine-derived polio occurs where vaccine coverage is low and there is poor sanitation.

Children who do get immunised are given a weakened form of the polio virus so their body can build up immunity to the disease. But they also then excrete the virus, which can then spread in the community.

There are currently outbreaks of vaccine-derived polio virus in 12 countries: Angola, Benin, Cameroon, Central African Republic, Chad, Democratic Republic of the Congo, Ethiopia, Ghana, Niger, Nigeria, Togo and Zambia.