The most dangerous outbreak of an emerging infectious disease since the appearance of H.I.V., in the early nineteen-eighties, seems to have begun on December 6, 2013, in the village of Meliandou, in Guinea, in West Africa, with the death of a two-year-old boy who was suffering from diarrhea and a fever. We now know that he was infected with Ebola virus. The virus is a parasite that lives, normally, in some as yet unidentified creature in the ecosystems of equatorial Africa. This creature is the natural host of Ebola; it could be a type of fruit bat, or some small animal that lives on the body of a bat—possibly a bloodsucking insect, a tick, or a mite.

Before now, Ebola had caused a number of small, vicious outbreaks in central and eastern Africa. Doctors and other health workers were able to control the outbreaks quickly, and a belief developed in the medical and scientific communities that Ebola was not much of a threat. The virus is spread only through direct contact with blood and bodily fluids, and it didn’t seem to be mutating in any significant way.

After Ebola infected the boy, it went from him to his mother, who died, to his three-year-old sister, who died, and to their grandmother, who died, and then it left the village and began moving through the human population of Guinea, Liberia, and Sierra Leone. Since there is no vaccine against or cure for the disease caused by Ebola virus, the only way to stop it is to break the chains of infection. Health workers must identify people who are infected and isolate them, then monitor everybody with whom those people have come in contact, to make sure the virus doesn’t jump to somebody else and start a new chain. Doctors and other health workers in West Africa have lost track of the chains. Too many people are sick, and more than two hundred medical workers have died. Health authorities in Europe and the United States seem equipped to prevent Ebola from starting uncontrolled chains of infection in those regions, but they worry about what could happen if Ebola got into a city like Lagos, in Nigeria, or Kolkata, in India. The number of people who are currently sick with Ebola is unknown, but almost nine thousand cases, including forty-five hundred deaths, have been reported so far, with the number of cases doubling about every three weeks. The virus seems to have gone far beyond the threshold of outbreak and ignited an epidemic.

The virus is extremely infectious. Experiments suggest that if one particle of Ebola enters a person’s bloodstream it can cause a fatal infection. This may explain why many of the medical workers who came down with Ebola couldn’t remember making any mistakes that might have exposed them. One common route of entry is thought to be the wet membrane on the inner surface of the eyelid, which a person might touch with a contaminated fingertip. The virus is believed to be transmitted, in particular, through contact with sweat and blood, which contain high concentrations of Ebola particles. People with Ebola sweat profusely, and in some instances they have internal hemorrhages, along with effusions of vomit and diarrhea containing blood.

Despite its ferocity in humans, Ebola is a life-form of mysterious simplicity. A particle of Ebola is made of only six structural proteins, locked together to become an object that resembles a strand of cooked spaghetti. An Ebola particle is only around eighty nanometres wide and a thousand nanometres long. If it were the size of a piece of spaghetti, then a human hair would be about twelve feet in diameter and would resemble the trunk of a giant redwood tree.

Once an Ebola particle enters the bloodstream, it drifts until it sticks to a cell. The particle is pulled inside the cell, where it takes control of the cell’s machinery and causes the cell to start making copies of it. Most viruses use the cells of specific tissues to copy themselves. For example, many cold viruses replicate in the sinuses and the throat. Ebola attacks many of the tissues of the body at once, except for the skeletal muscles and the bones. It has a special affinity for the cells lining the blood vessels, particularly in the liver. After about eighteen hours, the infected cell is releasing thousands of new Ebola particles, which sprout from the cell in threads, until the cell has the appearance of a ball of tangled yarn. The particles detach and are carried through the bloodstream, and begin attaching themselves to more cells, everywhere in the body. The infected cells begin spewing out vast numbers of Ebola particles, which infect more cells, until the virus reaches a crescendo of amplification. The infected cells die, which leads to the destruction of tissues throughout the body. This may account for the extreme pain that Ebola victims experience. Multiple organs fail, and the patient goes into a sudden, steep decline that ends in death. In a fatal case, a droplet of blood the size of the “o” in this text could easily contain a hundred million particles of Ebola virus.

Inside each Ebola particle is a tube made of coiled proteins, which runs the length of the particle, like an inner sleeve. Viewed with an electron microscope, the sleeve has a knurled look. Like the rest of the particle, the sleeve has been shaped by the forces of natural selection working over long stretches of time. Ebola is a filovirus, and filoviruses appear to have been around in some form for millions of years. Within the inner sleeve of an Ebola particle, invisible even to a powerful microscope, is a strand of RNA, the molecule that contains the virus’s genetic code, or genome. The code is contained in nucleotide bases, or letters, of the RNA. These letters, ordered in their proper sequence, make up the complete set of instructions that enables the virus to make copies of itself. A sample of the Ebola now raging in West Africa has, by recent count, 18,959 letters of code in its genome; this is a small genome, by the measure of living things. Viruses like Ebola, which use RNA for their genetic code, are prone to making errors in the code as they multiply; these are called mutations. Right now, the virus’s code is changing. As Ebola enters a deepening relationship with the human species, the question of how it is mutating has significance for every person on earth.

The Kenema Government Hospital, in Kenema, Sierra Leone, is a scatter of low yellow-and-red-painted cinder-block buildings with rusty metal roofs. It spreads down a hillside near the center of town, and, according to medical workers there, is normally bustling with patients and their families. The town sits in fertile, hilly country, dotted with small villages, ninety miles southwest of the place where the borders of Sierra Leone, Guinea, and Liberia converge in a triskelion. This border area was the cradle of the Ebola outbreak. For decades, the Kenema hospital has had a special twelve-bed unit called the Lassa Fever Ward and Research Program. Lassa fever is caused by Lassa virus, which is classified by virologists as a Biosafety Level 4 pathogen—lethal, infectious, typically with no vaccine and no reliable cure. In May of this year, the chief physician of the Lassa program, Sheik Humarr Khan, was watching out for Ebola, which, like Lassa, is a Level 4 pathogen. The virus had been spreading in Guinea and Liberia, but there had been no reported cases yet in Sierra Leone.