When a toddler in Meliandou came into contact with a fruit bat, a trail of devastation began that is far from reaching a conclusion

In December last year, near the village of Meliandou in southern Guinea, two-year-old Emile may have come into contact with one of the fruit bats that fly through west Africa’s skies, often gathering at dusk to roost in trees.

In that moment, Emile was destined for a place in history as Patient Zero, the unfortunate first victim of an Ebola outbreak that has already killed more than 3800 people and wreaked havoc across some of the most fragile countries in west Africa. The crisis has been described as “unparalleled in modern times”.

On 26 December, Emile fell ill with fever. He died two days later. Eight days after that, his three-year-old sister Philomena died. His mother Sia succumbed to the disease on 11 January, and his grandmother Koumba three days later, in a hospital in the nearby town of Guéckédou. Researchers believe that their funerals marked the next steps on the virus’s devastating sweep through Guinea and across the nearby borders into Liberia and Sierra Leone.

“Following the young boy’s death, the mysterious disease continued to smoulder undetected, causing several chains of deadly transmission,” according to the World Health Organisation (WHO). Villagers were frightened, and doctors were baffled. Some thought it was an outbreak of cholera. But no one was sure, and in this information vacuum the virus reached nearby towns and crossed borders.

The WHO identified Meliandou as lying in the outbreak’s “hot zone”: a triangle-shaped forested area where the porous borders of Guinea, Liberia and Sierra Leone converge. These countries have endured years of intertwined conflict over diamonds and other resources, leaving them with limited health services and a population used to crossing borders to seek work or refuge.

In a chart mapping the current outbreak’s spread, the WHO says a friend of Emile’s family from Sierra Leone contracted the disease and died in Kangama, which lies south-west of Guéckédou and across the border. The graphic’s Ebola tree of death also shows Koumba’s nephew travelling to the Guinean capital, Conakry, where he died on 5 February.

So began a chain of viral devastation, whose links represent individual lives cut short and ineluctable steps on Ebola’s path through west Africa.

In an article published in the New England Journal in April, researchers also tracked the spread of the outbreak, saying an infected health worker extended the disease’s reach beyond Guéckédou to surrounding areas.

“On 10 March 2014, hospitals and public health services in Guéckédou and Macenta alerted the ministry of health of Guinea and – two days later – Médecins sans Frontières about clusters of a mysterious disease characterised by fever, severe diarrhoea, vomiting and an apparent high fatality rate,” the researchers wrote.

On 23 March, the WHO published a formal notification of an Ebola outbreak in Guinea on its website. By 8 August, it declared the epidemic to be a “public health emergency of international concern”.

That month, a team of European and African researchers confirmed the hypothesis that the outbreak was caused by a toddler’s contact with a single infected bat.

The 17 experts spent three weeks around Meliandou, interviewing residents and capturing bats and other animals. They concluded that the disease was spread by colonies of migratory fruit bats.

Today, more than nine months after Emile’s death, exhausted and often ill-equipped health workers are still battling the virus in Sierra Leone, Guinea and Liberia. Smaller outbreaks were identified in Senegal and Nigeria but were swiftly brought under control. The first diagnosed case of Ebola in the US was announced on 30 September.

On 6 October, Spanish authorities said that a nurse who treated a Spanish priest – who had contracted Ebola in Sierra Leone and subsequently died in Madrid – had tested positive for the disease. She became the first person to contract the disease outside west Africa.

• This article was amended on 13 October 2014 to remove a statement that a two-year-old child in Meliandou, later identified as west Africa’s first case of Ebola, was bitten by a bat.