There was no bright side to any of the population becoming immune. It arsed Barty off to no end knowing that something wasn't working perfectly. He'd have to see to it that something was done about it that did not end up with half the population dead. If everyone was dead, there was no one for his Master to rule.
"Kieran MacGilly. He works for the Wizarding Wireless Network and has been causing quite a ruckus on the air. We have successfully dismantled the station, but I want a more permanent fix." He didn't care how she did it, he just knew she'd get it done.