ON MARCH 25th the World Health Organisation (WHO) reported a rash of cases of Ebola in Guinea, the first such ever seen in west Africa. As of then there had been 86 suspected cases, and there were reports of suspected cases in the neighbouring countries of Sierra Leone and Liberia as well. The death toll was 60.

On October 15th the WHO released its latest update. The outbreak has now seen 8,997 confirmed, probable and suspected cases of Ebola. All but 24 of those have been in Guinea (16% of the total), Sierra Leone (36%) and Liberia (47%). The current death toll is 4,493. These numbers are underestimates; many cases, in some places probably most, go unreported.

This all pales, though, compared with what is to come. The WHO fears it could see between 5,000 and 10,000 new cases reported a week by the beginning of December; that is, as many cases each week as have been seen in the entire outbreak up to this point. This is the terrifying thing about exponential growth as applied to disease: what is happening now, and what happens next, is always as bad as the sum of everything that has happened to date.

Exponential growth cannot continue indefinitely; there are always barriers. In the previous 20 major outbreaks of the disease since its discovery in 1976, all of which took place in and around the Democratic Republic of the Congo, the initial rapid spread quickly subsided. In the current outbreak, though, the limits have been pushed much further back; it has already claimed more victims than all the previous outbreaks put together.

Grim reckoning

There are two reasons for this. Those earlier outbreaks were often in isolated places where there are few opportunities for transmission far afield—the transfer of the virus between a wild animal and a human that sets off all such outbreaks is more likely off the beaten track. And they were mostly recognised quickly, with cases isolated and contacts traced from very early on; one was stopped this way in Congo in the past few months. The west African outbreak has broken through the barriers of isolation and into the general population, both in the countryside and the cities, and it was up and running before public-health personnel cottoned on. There is no reason to expect it to subside of its own accord, nor to expect it to come under control in the absence of a far larger effort to stop it.

Trying to be precise about how bad things could get, absent that effort, is not possible. This is not just because the actual number of cases is not well known. The rate at which cases give rise to subsequent cases, which epidemiologists call Rο, is the key variable. For easily transmitted diseases Rο can be high; for measles it is 18. For a disease like Ebola, much harder to catch, it is lower: estimates of Rο in different parts of the outbreak range from 1.5 to 2.2. Any Rο above 1 is bad news, though, and seemingly small differences in Rο can matter a lot. An Rο of 2.2 may sound not much bigger than an Rο of 1.5, but it means numbers will double twice as fast.

And Rο is not a constant. It depends both on the biology of the virus, the setting of its spread (city or country, slum or suburb) and the behaviour of the people among whom it is spreading. Over the course of the crisis the second two factors are bound to change as the virus moves to different places and as people start to adapt. Given high rates of mutation, which bring with them the possibility of evolutionary change, it is possible that the first could change, too. Peter Piot, one of the researchers who first identified the Ebola virus in 1976, stresses that the course of an outbreak does not always follow smooth curves; it can stutter and flare up. None of this complexity, though, offers much reassurance. While doubtless imperfect, plausible model-based extrapolations such as a recent one from America’s Centres for Disease Control and Prevention (CDC) suggest, in the absence of intervention, that there could be 1.4m cases in west Africa in the next three months.

Not that Ebola will necessarily be contained in west Africa. Despite it having infected health-care workers in America and Spain, and worries that one of those Americans could have passed it further, public-health experts are largely confident that outbreaks can be contained in countries with robust medical systems and the ability to trace contacts. But transfer to other places with poor health systems might allow the virus to take hold in new cities. Especially if it makes inroads into Nigeria, where one set of cases has been successfully controlled, the virus could travel on to India, rich in slums with poor health care, or China, where infection control in hospitals can be worryingly lax.

The steps to avert such a cataclysm are reasonably clear: cases must be identified quickly, patients isolated and their contacts traced; changes in behaviour which reduce transmission rates must be encouraged through education campaigns and community action. The difficulty is doing all these things quickly and on a large scale. Modelling suggests that getting 70% of the sick into settings that reduce transmission of the virus—clinics, treatment centres or safe settings for treatment in the community—would bring things under control. That is a tall order.

The three countries currently afflicted are all exceedingly poor and plagued by various levels of instability and dysfunction. Guinea, the only one to have avoided civil war following independence, has been plagued by military coups and civil strife. In Sierra Leone many public institutions had only just started to be rebuilt after the civil war that finished over a decade ago. The wounds of Liberia’s civil war are fresher and deeper. Foreign peacekeepers maintain public security; many institutions barely exist. At the start of the crisis the countries had only a few hundred doctors between them. Many of those doctors, including Sheik Umar Khan, who led Sierra Leone’s response, have since died of the disease. In an echo of the way that, inside the body, it targets the immune system first, in the community Ebola hits health-care workers hardest.

Providing the infrastructure for a better response is thus a matter for outsiders. Some help has come from governments, some from non-governmental organisations such as Médecins Sans Frontières (MSF), an NGO which has provided about two-thirds of the isolation beds used to treat Ebola patients so far. Expansion moves apace. Beneath the looming Peninsula Mountains to the south of Sierra Leone’s capital, Freetown, the sleepy village of Kerry Town is the scene of frantic activity, as more than 200 construction workers sweat through the night to complete the first of six Ebola clinics to be set up in the country by the British army (some snatch a nap on the table in the morgue). Solar panels are being installed, a borehole drilled for water, a concrete access road laid to link up with the coastal highway. The centre will hold 90 beds, with an additional 12 set aside in a ward for the health workers. A military spokesman says the site should be completed by the end of the month.

For a few billion more

The 70-bed Ebola treatment unit in Bong County, Liberia, was built by Save the Children, an NGO; it took about four weeks to build. Chris Skopec of International Medical Corps, the NGO that runs it, describes it as “something in between a tent and a concrete structure”. But it has all the necessary features: quarantine rooms, decontamination areas and large toilet spaces (patients suffering from vomiting and diarrhoea may pass out). It is close enough to villages for people to reach, not so close for them to protest at its presence.

These efforts are impressive. Liberia’s capacity to treat Ebola victims has nearly doubled in the past two weeks, and America has promised to build 17 100-bed units in the coming months. However, thanks again to the power of exponential growth, if the number of beds can be doubled only at more or less the same rate that the virus doubles the number of cases, the disease’s head start will grow ever more overwhelming. For the caseloads predicted for late November and December, the 70% treatment level seen as needed to bring things under control corresponds to tens of thousands of beds.

For a sense of the resources required to raise the tempo, consider that the 70-bed facility in Bong cost $170,000 to build. It needs a staff of 165 to treat patients and handle tasks like waste management and body disposal. It is likely to go through nearly 100 sets of overalls, gowns, sheets and hoods per day. The monthly cost of running the unit comes out at around $1m, which is about $15,000 a bed. The WHO puts the costs of a 50-bed facility at about $900,000 a month. These figures suggest that a 100,000-bed operation would cost in the region of $1 billion-$2 billion a month.

Various countries have promised substantial aid, but not yet on that scale. America has pledged $350m and set aside another $1 billion to fund the activities of its soldiers in the area. Britain has committed $200m. The World Bank has set up a $400m financing scheme; the first $105m reached the governments of the affected countries in just nine days. The UN, of which the WHO is part, has taken in about a third of the $1 billion it says it needs to fund its own efforts in the region; all told, though, Ban Ki-Moon, the UN secretary-general, sees a need for much more than that—“a 20-fold surge” in assistance.

Money is of little use without staff. China has sent some 170 medical workers to the affected countries. Cuba, long focused on medical work overseas, has sent a similar number, and has plans for 300 more. Others have been less forthcoming. The facilities America’s soldiers are building will require a staff in the thousands; despite being trained for biological and chemical warfare American troops will not be among them. Last month MSF rejected a pledge of $2.5m from the Australian government, demanding Australian doctors instead. Australia demurred.

While there are medical volunteers from overseas, Ebola is a harder sell than other crises. David Wightwick of Save the Children says that in the aftermath of Typhoon Haiyan hitting the Philippines there weren’t enough seats on the planes for all of the international volunteers—but when he asked 28 logisticians to travel to the affected countries, 21 said no. Nevertheless, Bruce Aylward, who is overseeing the WHO’s response in west Africa, says an increasing number of NGOs and foreign governments are now looking to deploy staff to the region.

Funerals and friends

With the number of sick outstripping the capacity of the treatment centres, more care is being moved into the community—which requires a reliance on local people with rudimentary training that Dr Aylward says would have been considered heretical in earlier Ebola outbreaks. The isolation is needed because it is when people are at their sickest that they are at their most contagious. The virus is transmitted by direct contact with body fluids and excreta: the most infectious are blood, faeces and vomit, which are most likely to be contacted when the sickness is at its height.

Ebola in graphics: The data behind the crisis The minimum basis for community care is to have two structures, which might be tents or shacks, set aside for suspected and confirmed cases. The carers would not be health workers, but trained community members with proper protective gear. People who have already survived the disease appear to have subsequent immunity and could be well employed in such settings; the dependability and duration of their immunity is not fully clear, and they would still need to follow safety procedures, but they would run less risk. The sick would be given only rudimentary care, not least because communities often lack reliable electricity or water supplies. Most will go into such facilities with a fever brought on by something more common but less lethal than Ebola, like malaria; there are not yet tests for Ebola in the field that would keep such cases out. Some will die who otherwise would not; the hope is, though, that 70% will come out alive. If only people with Ebola went in, that figure would be more like 30%. Such care units are being piloted in Sierra Leone and Liberia, and in many cases there may seem little if any alternative. Still, Christopher Stokes of MSF urges caution. If the locals are not properly trained, he warns, “you can amplify the epidemic, because they will feel confident in being around patients and they will catch it themselves and infect others.” The fact that the virus succumbs very readily to disinfectants such as bleach is welcome, but it will not help unless the disinfectants are used thoroughly and consistently. Mr Stokes prefers decentralisation, “where you go closer to the community with smaller units [of about 30 beds], but with properly trained staff, which MSF has done in Guinea.” The approach worked well; at one point the outbreak in Guinea seemed almost to have been stopped. But economies of scale suggest that most new treatment centres will be a lot bigger, with some offering 100 beds or more.

Isolation reduces transmission. So can behaviour change, on which governments, lacking the wherewithal for much else, have concentrated their response, and which experts like Dr Piot see as the heart of the problem. Much of the focus to date has been on the burial of the dead. Those who have died remain, for a while, very infectious, and funerals can bring people from some distance. Six months into the epidemic a WHO study concluded that 60% of all cases in Guinea were linked to traditional burial practices that involve touching, washing or kissing the body. All the earliest cases of the disease in Sierra Leone appear to have been contracted at a single large funeral in Guinea, one which was also crucial in reigniting the epidemic in that country.

Now the traditions and beliefs that place such reverence on the treatment of the dead are being regretfully put aside by many; funerals that were once vibrant social events are in some places becoming practical exercises in the burial of body bags. “My aunt was taken away like a broken fridge and there was no other way,” says Charles Washington, a hotel worker in Liberia. But there are still traditional, dangerous funerals going on. There is more to be done through community engagement to reduce dangerous practices and to make rituals safer. Involving churches, traditional healers and the region’s secret societies more would bolster this and other interventions, such as those which help people to understand how the disease is transmitted.

Leaflets, placards and public-service announcements tell citizens in all three countries how to protect themselves through hand-washing and minimising contact with the ill. Sierra Leone went as far as locking down the country for three days during which officials and volunteers went house-to-house to educate people as well as search for hidden outbreaks. In Liberia and Sierra Leone, Ebola is a popular topic on the radio, which is how most people get their information. The broadcast advice is sensible and sometimes musical: “Ebola is Real” by F.A., Soul Fresh and DenG is proving popular in Liberia. Public buildings have temperature checks at the entrance; many also have chlorine baths for hands and shoes. People are aware of the danger surrounding them; many speak of little else.

Mobile phones also spread useful information—and may provide vital data to health workers. The CDC is tracking the location of people who call helplines in order to see where the disease is spreading. A Swedish NGO called Flowminder has captured people’s movements in the region using mobile-phone records.

Change in behaviour is real, but by most accounts it is patchy. Some people continue to believe that Ebola can be fought with animist remedies or witchcraft. Much to the frustration of a beleaguered cemetery keeper, people still wander through his graveyard in Freetown on their way to work, oblivious to the risks. Taxi drivers may disinfect their vehicles more, but in Liberia they chafe against new rules limiting passenger numbers. When livelihoods are at stake, onerous rules will be broken.

And crafting clear messages is hard. Dr Piot points to juxtaposed posters saying first that there is no cure and second that the infected should get to treatment centres. Despite such mixed messages, early fears that treatment centres would be shunned as death traps have not, in the main, come true; many centres are full. But this leads to another problem. Is it sensible to encourage sick people to take long journeys with no bed at its end? Progress depends not just on more beds, but on more local information on where to find them, and what to do if they are not available. Communities and the people from whom they seek advice need to be informed enough for such responses. They need to be involved in ways that help them decide how to reorganise their lives. Add that to the list of things easier said than done.

Veni, vidi, vaccini

Changing behaviour could slow the spread of the virus; a vaccine could potentially stop it. In large part because of worries that Ebola might be used as a biological weapon, vaccines that protect lab animals against the virus were already on the shelf when the outbreak began. Two are now being tested for safety in humans, and one of them could, if it is safe, be tested for efficacy quite soon, most likely in health-care workers in west Africa. Its maker, GlaxoSmithKline, could have 10,000 doses ready in a few months. Meanwhile thought is going into how to scale up production of any vaccine that proves successful. The ideal would be to come up with some mixture of direct payment to companies and guaranteed purchases that would mean copious stocks were available the moment the good news came through.

The other vaccine in trials might possibly, on the basis of animal tests, have the added benefit of helping those infected fight the virus as well as keeping the uninfected safe. At the moment there is a striking lack of such therapies: ZMapp, a cocktail of antibodies that has worked in animals, is of unproven efficacy and exhausted supply. A lower-tech alternative is to use blood serum from recovered patients, which contains the antibodies that helped them fight the virus. Such blood would have to be screened for other pathogens and matched to the recipients’ blood type, but WHO experts have been guardedly optimistic about the idea.

Even if treatment centres are hugely expanded, people’s behaviour changes radically and a vaccine proves effective, the damage already done to the region is huge. The patterns of work and food supply are already disrupted. Some farmers have abandoned their fields because they wrongly fear being infected by water in irrigation channels; some in cities are panic-buying. Salaries to public employees are not secure. The World Bank warns that Liberia’s rubber production, a big export earner, could fall drastically.

For now mounting deaths, understandable confusion and increasing economic dislocation have not caused widespread civil unrest. But many fault their governments for not protecting or preparing them better for the epidemic, and the grudges that animated past civil wars and coups sleep lightly. Few diplomats see a return to the bad old politics as out of the question; Filipino UN peacekeepers in Liberia have been withdrawn by their government. If civil order breaks down, the epidemic will get still worse.

Even if things do not fall apart, there is the most uphill of struggles ahead. Dr Piot cautions that an Ebola outbreak is an all-or-nothing affair; it is only over when the last patient is either dead or fully recovered. When it has struck on this scale, the challenges that remain after that will still be huge; whole public-health infrastructures will need rebuilding. But first there is a mountain to climb.