FOR months the world has sat largely idle as an Ebola epidemic has marched steadily from the remote jungles of Guinea to the slums of Liberia, and beyond. On September 16th that changed. Barack Obama announced the largest humanitarian deployment by America’s armed forces to fight an infectious disease. Saying that the epidemic “is not just a threat to regional security—it’s a potential threat to global security if these countries break down”, the president began the process of sending some 3,000 American troops to set up treatment centres with 1,700 beds and to train local health workers.

The dispatch of troops to west Africa may seem an odd priority when American forces are preparing to confront jihadists in Iraq and Syria and are stretched thin elsewhere. Ebola is a disease that is usually absent from human populations, has been quickly stamped out in the past and in its worst recorded outbreak has thus far caused 3,000 known deaths (see article). Moreover it is unlikely to spread widely in rich countries with good health-care systems. Set against killers such as HIV, the virus that kills some 1.6m people a year, or tuberculosis (TB), which takes another 1.3m lives, an expensive fight against Ebola may seem a misallocation of resources.

Yet Ebola is now growing exponentially, with the number of new cases roughly doubling every three weeks or so. In Monrovia, the capital of Liberia, it is thought to be doubling every two weeks. Previous outbreaks were usually in rural villages where it was easier to contain. At this rate of progress, small numbers quickly become big ones, and there is a real risk of the disease spreading to cities such as Lagos, which is home to more than 10m people. The longer Ebola is allowed to replicate in humans, the greater the risk that it will become more contagious. Some virologists fret that it might even acquire the ability to be transmitted through the air by coughs and sneezes. Although this seems unlikely, nobody wants to find out just how quickly Ebola can adapt to humans.

America’s response is the first by a government on a large scale. Until now the burden has been carried by charities such as Médecins Sans Frontières (MSF), which has 2,000 staff in the affected countries. Yet even America’s large commitment may not be enough to get ahead of this epidemic in Liberia, the country most affected. By the time the troops actually get there, the situation in Liberia could be far worse.

Gloves and masks needed

Elsewhere, the response falls far short of what is needed. Sierra Leone, the second-worst-affected country, has received far less help from governments: China is sending 174 people and mobile laboratory teams, Cuba is sending a similar number and Britain will set up a hospital with 62 beds. France is sending 20 people to the region (though it is expected to announce that it will build a hospital).

Two things are urgently needed. The first is the rapid provision of basic (and, frankly, cheap) protective gear such as gloves, gowns, surgical masks and disinfectant. Domestic health systems in affected countries have crumbled as nurses and doctors have fallen ill or died for lack of basic gear.

The second need is for trained staff to run the treatment centres and work in them. Poorly run ones with weak infection controls may hasten the spread of the disease. Both are needed soon, as the cost of halting Ebola’s spread is also rising exponentially. In August the World Health Organisation estimated that it would take nine months and cost $490m to contain Ebola. Now it reckons the cost has risen to over $1 billion. The longer the world prevaricates, the harder and costlier it will be to contain this outbreak.