THE first reported case in the Ebola outbreak ravaging west Africa dates back to December 2013, in Guéckédou, a forested area of Guinea near the border with Liberia and Sierra Leone. Travellers took it across the border: by late March, Liberia had reported eight suspected cases and Sierra Leone six. By the end of June 759 people had been infected and 467 people had died from the disease, making this the worst ever Ebola outbreak. The numbers keep climbing. As of November 18th, 15,351 cases and 5,459 deaths had been reported worldwide, the vast majority of them in these same three countries. Many suspect these estimates are badly undercooked.
An interactive map of the virus's current global reach is here.
The outbreak continues to claim lives, but there are glimmers of good news. The number of new cases reported each week in Guinea and Liberia has stabilised (in Liberia, the flattening out has come after a decline in new cases between mid-September and mid-October). The pattern in Sierra Leone is much less encouraging, however: 533 new confirmed cases were reported in the week to November 16th, the highest number yet.  The chart above shows numbers from both the World Health Organisation's regular situation reports and from patient databases, which tend to be more accurate but are less complete for recent weeks.
The rate at which cases give rise to subsequent cases, which epidemiologists call R0, is the key variable in the spread of Ebola. For easily transmitted diseases R0 can be high; for measles it is 18. Ebola is much harder to catch: estimates of R0 in different parts of the outbreak range from 1.5 to 2.2. Although there are some signs that the virus is gradually being brought under control in Guinea and Liberia, any R0 above 1 is bad news. The very high mortality rate of the disease, estimated at 70% in this outbreak, means that Ebola can quickly claim more lives than other, more established killers.
The inadequacies of the health-care systems in the three most-affected countries help to explain how the Ebola outbreak got this far. Spain spends over $3,000 per person at purchasing-power parity on health care; for Sierra Leone, the figure is just under $300. The United States has 245 doctors per 100,000 people; Guinea has ten. The particular vulnerability of health-care workers to Ebola is therefore doubly tragic: as of November 18th there had been 588 cases among medical staff in the three west African countries, and 337 deaths.