From a reader (does anyone read this blog?). Thanks Ben!!!
‘Shortly after the Aschen arrived, there was a flu pandemic. The Aschen distributed a vaccine that saved them — but it apparently had a side effect. “Vaccine causes sterility.”‘
‘As Daniel points out, the Volians went from a thriving metropolis of millions to an agrarian civilization of thousands — after the Aschen supposedly saved them.’
They do the same to Earth by giving them an anti-aging vaccine. Humans learn it causes sterility too late. SG1 have to time-travel to undo the plot.