The study, published in the New England Journal of Medicine, claims that the disease started to spread when the toddler died after showing symptoms of fever, vomiting and diarrhea. The boy's mother, sister and grandmother all died a shortly after. They were all treated by a healthcare worker in Guéckédou, who also became infected.
Doctors are not sure how the child contracted the disease, but they suspect handling raw fruit bat meat, or being injected with an unsanitized needle could be the cause.
"We suppose that the first case was infected following contact with bats," Sylvain Baize, the author of the study, told The New York Times. "Maybe, but we are not sure."
According to the paper, the doctors traced the disease by following the timeline of the outbreak and interviewing patients and healthcare workers until they linked it to the source.
Guéckédou lies at the epicenter of the disease outbreaks in Guinea, Sierra Leone and Liberia. The World Health Organization has declared the outbreak a public health emergency.
Nearly 1,000 people have died from Ebola.