Ten years after Sars, now we have Mers

Margaret Chan, director general of the World Health Organization, 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 30 lives. But it is not what it has done that is worrying – it is what it may do.

Addressing the World Health Assembly in Geneva last week, Dr 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. The novel coronavirus is not a problem that any single country can manage by itself."

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 8,000 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. But this is no ordinary cold. In cases seen so far, the Mers virus 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, after travelling from Saudi Arabia, were so ill they 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.

Until now, 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".

As long as that remains the case, there is little need to worry. But as Dr Chan explained, we do not know where the virus originated – bats have been suggested, but not confirmed – 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.

While Mers is causing alarm in the Middle East and Europe, health experts in the Far East are worrying over a different threat – from a new strain of flu that could be a harbinger of the next pandemic.

H7N9 has been described as the "nastiest virus in humans in years". It is deadly, resistant to anti-flu drugs and poses a "serious threat" because of its potential to mutate and create a new pandemic strain, according to Professor John McCauley, director of the WHO's collaborating centre for flu surveillance in the UK.

There have been 132 laboratory confirmed cases and 37 deaths, says the WHO. The latest case is a six-year-old boy who became ill in Beijing on 21 May. The virus has spread to half a dozen provinces in China and to Taiwan but appears to have slowed in the past month.

Its source has not yet been identified, but a study published in The Lancet suggested H7N9 was a mix of at least four viruses that originated in ducks and chicken. Many of the cases started around Shanghai, which responded by closing poultry markets. A sharp fall in the number of new infections followed.

The big fear is of human-to-human transmission. There is no evidence that it has happened so far, but neither the WHO nor China has ruled it out. The Taiwanese case had no known exposure to poultry.

"This is one of the most lethal influenza viruses we have seen so far," said Keiji Fukuda, WHO assistant director general. "It is an unusually dangerous virus for humans."

Whenever a flu virus jumps from its normal host in birds or animals to humans, it sets alarm bells ringing. They are ringing now among scientists across the world.