A genomic analysis has narrowed down where and when a deadly bacterium jumped from pigs to humans: in Europe in the 1960s or 1970s.

The bacterium Streptococcus suis has long been known as a pig pathogen. But, since 1998, it has caused three large outbreaks in people in China, and a sharp increase in human infections globally, making it a serious public-health concern.

To trace the evolution of S. suis, Jinquan Li at Huazhong Agricultural University in Wuhan, China, Ye Feng at Zhejiang University in Hangzhou, China, and their colleagues analysed 1,634 S. suis genomes collected from pigs and humans across 14 countries over 36 years. The genomes fell into three distinct groups, one of which was found mostly in humans. This group included strains that caused severe disease in zebrafish, which the authors used as a model of human disease.

Some of those strains had been collected from pigs that showed no signs of illness, suggesting that asymptomatic animals might have acted as carriers, spreading the bacterium to people. The researchers traced the human group’s origins to Europe during the 1960s and 1970s, when the continent was exporting pigs worldwide.