The man died two hours after arriving. Tests later showed he had been positive for Ebola. Untold numbers of health care workers and their subsequent patients had been exposed to the disease.

Gloves, in short supply at the hospitals, were selling for 50 cents a pair on the open market, a huge sum for people who often live on less than a dollar a day. At homes where families cared for patients, even plastic buckets to hold water and bleach for washing hands and disinfecting linens were lacking.

Workers were failing to trace all patients’ contacts. The resulting unsuspected cases, appearing at hospitals without standard infection control measures, worsened the spread in a “vicious circle,” Dr. Mardel said.

Tracing an Epidemic’s Origins

As is often the case in Ebola outbreaks, no one knows how the first person got the disease or how the virus found its way to the region. The virus infects monkeys and apes, and some previous epidemics are thought to have begun when someone was exposed to blood while killing or butchering an infected animal. Cooking will destroy the virus, so the risk is not in eating the meat, but in handling it raw. Ebola is also thought to infect fruit bats without harming them, so the same risks apply to butchering bats. Some researchers also think that people might become infected by eating fruit or other uncooked foods contaminated by droppings from infected bats.

Once people become ill, their bodily fluids can infect others, and they become more infectious as the illness progresses. The disease does not spread through the air like the flu; contact with fluids is necessary, usually through the eyes, nose, mouth or cuts in the skin. One drop of blood can harbor millions of viruses, and corpses become like virus bombs.

A research team that studied the Guinea outbreak traced the disease back to the 2-year-old who died in Guéckédou and published a report in The New England Journal of Medicine. He and his relatives were never tested to confirm Ebola, but their symptoms matched it and they fit into a pattern of transmission that included other cases confirmed by blood tests.

But no one can explain how such a small child could have become the first person infected. Contaminated fruit is one possibility. An injection with a contaminated needle is another.