Nigeria has announced an emergency mass polio vaccination campaign in the north-east after two new cases emerged.
They were the first incidences of the highly infectious disease in Africa for two years.
The government said polio paralysed two children in Borno state, a part of Nigeria where Boko Haram militants have hindered health campaigns.
The development is seen as a major setback for Nigeria, which was on track to be declared polio free in 2017.
In a statement, the government said that one million children would be immunized in the affected areas in Borno and a further four million will also be targeted in neighbouring states.
But it did not say exactly when the campaign will start.
Polio is a viral disease that usually affects children and can only be prevented through immunisation.
It is spread by poor sanitation and contaminated water.
What is polio?
- Polio, or poliomyelitis, mainly affects children aged under five
- It is a highly infectious disease caused by a virus. It invades the nervous system and can cause total paralysis in a matter of hours
- Initial symptoms include fever, fatigue, headache, vomiting, stiffness of the neck and pains in the limbs
- One in 200 infections leads to irreversible paralysis. Among those paralysed, 5% to 10% die when their breathing muscles become immobilised
- Today, only two countries – Afghanistan and Pakistan – remain polio-endemic, down from more than 125 in 1988