This spring in West Africa, a muttering emergence of the Zaire species of Ebola virus turned explosive. As of this writing, more than thirteen hundred cases of Ebola-virus disease have been officially reported in Guinea, Sierra Leone, and Liberia. The virus is spreading, uncontrolled, in widening chains of infection, which include cities—something never seen before. In Liberia, parts of the medical system have effectively collapsed. Some hospitals and clinics have been abandoned, while others have become choked with Ebola patients. The hospitals of Monrovia, the capital of Liberia, are full of Ebola patients and are turning away new patients, including women in childbirth. American Ebola experts in Monrovia are hearing reports that infected bodies are being left in the streets: the outbreak is beginning to assume a medieval character. People sick with Ebola are leaving Monrovia and going into the countryside to search for village faith healers, or to stay with relatives. In Sierra Leone, in the town of Kenema, eighteen doctors and nurses who had been working in the Lassa/Ebola ward have contracted Ebola, and at least five have died. They had been working in biological-hazard suits, yet they got sick anyway. People are wondering if the virus could spread to Europe or the United States, but the more immediate question is whether it could infect a whole lot more people in Africa.

A particle of Ebola-Zaire virus is made of only ten proteins, locked together in what looks like a tangle of string. Despite its extreme simplicity as an organism, when Ebola strikes a human it becomes a killing machine, the biological equivalent of a steel axe. The virus is transmitted from one person to the next through contact with blood or other bodily fluids. The symptoms of the disease start out looking like those of malaria: the patient runs a fever and feels weak. Ebola patients proceed to vomiting and diarrhea, which sometimes turns black; and they can develop hiccups. Fewer than half the patients in this outbreak have shown signs of hemorrhage: pinpoint droplets of blood can sometimes glisten on the rims of the eyelids. Around sixty per cent of the victims have died.

In July, as the outbreak gathered force, Daniel Bausch, an American doctor and Ebola expert, arrived in Freetown, Sierra Leone, and proceeded on to the Lassa/Ebola ward in Kenema, a facility that he helped set up. The hospital is a cluster of small cinder-block buildings in the center of town. He put on personal protective equipment, known as P.P.E.—a type of biohazard gear that consists of a Tyvek whole-body suit, a Tyvek hood with an opening for the eyes, safety goggles, a breathing mask over the mouth and nose, two pairs of nitrile gloves, a plastic apron, and rubber boots—and he walked into one of the Ebola wards, a makeshift structure with walls made of plastic film. There he found the director, Dr. Sheik Humarr Khan, and a nurse wearing biohazard suits and taking care of thirty Ebola patients. “The floor was splashed with blood, vomitus, feces, and urine,” Bausch said recently. Patients in the throes of Ebola often fall out of bed. “You need a whole team to decontaminate the bed and lift the patient up off the floor and put him safely back in bed.” Khan and the nurse were overwhelmed.

Some Ebola nurses had stopped coming to work: they had been working twelve-hour days, in biohazard suits, and they were supposed to be earning an extra thirty dollars a week in hazard pay, but the government of Sierra Leone had not provided it. Not unreasonably, many nurses had stopped showing up.

Last month, Joseph Fair, an American doctor who had worked in the ward, arrived to help in Kenema. He considered Khan one of his best friends, and told me that he regarded the chief nurse, a woman in her fifties named Mbalu Sankoh, who’d helped run the Kenema ward for twenty-five years, “as more or less a mom to me.” He embraced Mbalu when they met, and he warmly greeted another old friend, a senior nurse named Alex Moigboi.

Two days after Fair’s arrival, Nurse Mbalu developed a fever. She placed herself in a bed in the Kenema hospital and gave herself an I.V. drip, and told people that she thought she had malaria. Eight days later, she died. Since he had embraced her, Dr. Fair began testing his blood for Ebola virus. Alex Moigboi died shortly afterward.

For weeks, Dr. Fair has been going to funerals in Kenema. “I just couldn’t keep it together at Mbalu’s funeral,” he said last week. He was in Freetown, and had just returned from the funeral of Dr. Khan. “The ones who have passed are the only ones who would go into the Ebola ward,” Fair said. “Now we have a huge void.” He recounted how Dr. Khan, as he lay dying in the ward he had run for ten years, had whispered to a fieldworker, “James, I am going. You have to carry on.” ♦