There appeared to be two waves of Spanish flu during 1918 in South Africa - one mild and one severe - and they occurred almost simultaneously, Professor Howard Phillips wrote in the South African Historical Journal, 20 (1988).
The first wave came through Durban early in September 1918 and was carried to the rest of Natal and the Witwatersrand rapidly. By the 22nd of September thousands of workers on three Johannesburg Gold Mines were infected and within a fortnight the flu had spread to the general population.
The second more virulent strain came from Freetown in Sierra Leone to the Cape on 13 September 1918, spread through the Cape, the Free State and the Western Transvaal with devastating effect. There were two broad belts along which the epidemic moved from the Cape - the first in a north-easterly direction as far as the Western Transvaal, and the second to De Aar and then towards the south east to the Ciskei and the Transkei.
It is thought that those who contracted the first infection were partially immunised against the second more fatal one, which explains low mortality rates in Natal and certain areas of the Transvaal.
What is the flu vaccine?
Everything about flu vaccination
What is the difference between flu and a cold?