The first reported case of this flu occurred at Camp Funston, Kansas on the 4th of March 1918. On the 11th of March at Fort Riley in the same state a soldier reported sick with flu symptoms. By noon there were over a hundred cases and by the end of the week 500. It spread rapidly, but was reasonably mild.
By August 1918, however, the virus had transformed itself into something lethal and appeared almost simultaneously in France, Sierra Leone and Boston.
In New York in mid-October 1918, the death rate for a single day was 700 times higher than normal, with 851 people dying on one day. Within months, across the world, it had killed 20 million people, who died horribly, 'literally drowning as fluid filled their lungs'.
Sources for the whole section on the 1918 epidemic in South Africa: SA Railways and Harbour Magazine, December 1918; Phillips, Howard. South Africa's worst Demographic Disaster: The Spanish Influenza Epidemic of 1918. (South African Historical Journal, (20), 1988.
Map: the spread of the 1918 flu
How do people get flu?
A day in the life of a flu virus - thoughts and confessions