The fever. That's how you see the Kindestod. That's why Celia could see it. That's why Ryan still can. It's the only way. -But how can you fight this thing when you have a 107 degree temperature ? -I guess we'll find out. -Buffy ! -Willow, I'm doing this-- -That's 100% pure. It'll kill you in an instant. -They should put that on the label. -Here. It needs to be diluted.
―Buffy to Willow[src]

The Flu Virus was used by Buffy who reaches inside Dr. Stanley Backer's refrigerator and pulls out a red-labelled vial containing the virus. She expressly drink it for to see and kill Der Kindestod.