Why is polio a public health emergency?
At first glance it might seem odd for the spread of polio to be declared an international public health emergency.
There have been 68 recorded cases of wild poliovirus so far in 2014. Last year there were 417 cases.
Contrast that with the one million children under five who die from pneumonia each year or the 750,000 who die from diarrhoeal disease.
Like polio, most of those deaths are vaccine-preventable.
The only previous threat to have been accorded the same status by the WHO was the H1N1 swine flu pandemic in 2009.
The reason for the WHO declaration is the risk that the goal of polio eradication may be not be achieved.
Billions of dollars are spent each year on polio immunisation and the number of cases has plummeted since the late 1980s.
There have been important milestones: it is more than three years since the last polio case in India.
While the wild poliovirus continues to circulate, mass immunisations must continue in every country in the world.
If the disease was wiped out - like smallpox in the 1970s - then the money spent on polio immunisation could eventually be targeted elsewhere.
The declaration of a "public health emergency of international concern" is a measure of the potential threat to the eradication efforts.
The WHO wants all residents and long-term visitors to Pakistan, Cameroon and Syria to have been recently immunised and carry a certificate of vaccination.
All three countries have spread the virus across their borders this year during what is usually a low transmission season.
The virus has a higher risk of transmission during May and June.
There are several other countries infected with wild poliovirus which have not exported the disease - Nigeria, Afghanistan, Equatorial Guinea, Ethiopia, Iraq and Somalia.
The virus - which is shed in faeces - can spread rapidly from just one infected individual.
Most people who carry the virus show no symptoms, making it a very difficult disease to wipe out.
That is why it is so important that polio immunisation levels are maintained while the virus continues to circulate.
Polio eradication would be a huge achievement. A target date of 2018 has been set - but previous deadlines have come and gone.
The next six months may show whether that goal really is a realistic target.