Clusters of the disease popped up in early 2014 in these areas, with the initial patients suffering fever, vomiting and severe diarrhea, according to the report. Hemorrhaging was less frequent, the report noted.
In early March, the Ministry of Health in Guinea and Doctors Without Borders in Guinea were notified about the disease clusters.
Health investigators arrived that month and began tracing the disease by examining hospital documents and conducting interviews with affected families and villagers.
Ebola has now spread to Liberia, Sierra Leone and Nigeria, prompting global concerns.
The report about the emergence of Ebola in Guinea was authored by dozens of international doctors and researchers from institutions in France, Germany, Guinea, WHO and Doctors Without Borders.