A deadly version of an intestinal germ has spread through sub-Saharan Africa, helped by genetic mutation and diseases such as HIV that weaken the immune system and expose the body to infection.
The finding comes in a genetic comparison of variants of Salmonella Typhimurium, one of the Salmonella family of stomach bugs.
In a new study published by the journal, Nature Genetics, researchers say a new form of the germ emerged in the southeast of the continent 52 years ago, followed by a second wave, which came out of central Africa 17 years later.
The variant is the cause of an enigmatic disease called invasive non-typhoidal salmonella, which affects Africa far more than other continents and is suspected to be transmitted from human to human and kills between 22 and 45 per cent of those it infects.
Outside Africa, Salmonella without this variant tend to cause acute diarrhoea but the death rate is less than one per cent.