Dr Margaret Chan, director-general of the World Health Organisation, did not mince her words.
The deadly Sars-like virus that has spread in recent months from the Middle East to Germany, France and the UK, killing more than half of those it has infected, is a "threat to the entire world".
Since it emerged in Saudi Arabia in September last year, the new virus has spread to 50 people in eight countries and claimed 33 lives.
Saudi Arabia said last night that three more people had died.
Addressing the World Health Assembly in Geneva last week, Chan said: "We understand too little about this virus when viewed against the magnitude of its potential threat. Any new disease that is emerging faster than our understanding is never under control.
"These are alarm bells and we must respond."
Her words recall the panic that gripped the world when Sars - severe acute respiratory syndrome - appeared in 2003 and swept around the globe, infecting more than 8000 people and causing 800 deaths.
The WHO has called the new illness Mers - Middle East respiratory syndrome - reflecting its geographical origins. So far Mers has proved less infectious than Sars - but for how long?
The new illness is caused, as Sars was, by a coronavirus, the agent responsible for most common colds. Mers causes fever, pneumonia and breathing difficulties.
The worst-affected victims drown in their own secretions. Two of the three people who fell victim in the UK had to be treated on a machine that extracted blood from their body and oxygenated it artificially, to give their hearts and lungs a chance to recover.
All the known cases have occurred among patients with multiple health problems and low immunity.
The virus has spread only within hospitals and health centres or among families where there has been close contact.
There is, says the WHO, "no evidence of sustained human-to-human transmission".
But we do not know where the virus originated or how people are getting infected. Nor do we know if the cases identified represent the tip of an iceberg of milder cases, or the whole iceberg.