4. August 1918 saw simultaneous outbreaks in Europe, Ireland, North Africa, and the United States
In the late summer of 1918 outbreaks of the Spanish Flu, still not yet known by that name, occurred in Freeport, Sierra Leone, Boston, Massachusetts, Brest, on the coast of France, and throughout Ireland. The locations all had in common the fact that they contained ports visited by the ships of Great Britain; Boston for American provided war materiel and supplies; Freeport to supply the British West African troops against German African colonies; Ireland to supply troops and foodstuffs to the Western Front; and Brest as the main port for providing supplies to the British Army in France. American troops also landed at Brest, many carried by British ships.
The strain which appeared in August was far more virulent than that which had preceded it in the spring, spread more rapidly, and with a strong presence in busy seaports ensured itself the basis for continuing distribution. Seaports were all supported with busy rail networks, allowing the disease to be carried to other destinations before it had even made its appearance in the town supporting the port. Ireland was also connected to Great Britain with ferries, and the disease crossed into Great Britain along with ferry passengers and the returning British seamen and soldiers sent home to convalesce. As the summer of 1918 ebbed into the last autumn of the war, the flu pandemic erupted worldwide, appearing at every place where ships could find ports of call, as well as many where they could not.