UN warns of polio 'emergency'
The spread of polio is an international public health emergency that threatens to infect other countries with the crippling disease, according to the World Health Organisation.
Polio usually strikes children under five and is usually spread via infected water. There is no specific treatment or cure, but several vaccines exist.
Experts are particularly concerned the virus continues to pop up in countries previously free of the disease, such as Syria, Somalia and Iraq - where civil war or unrest complicates efforts to contain the virus.
Some critics say the rapid spread of polio could unravel the nearly three-decade effort to eradicate it.