The 'Now' rockers - Hayley, Jeremy Davis and Taylor York- called off their performance in Salt Lake City, Utah, to allow the frontwoman to regain good health after being struck with a sore throat last Tuesday (30.04.13) and then food poisoning.
Hayley took to her Tumblr to offer her sincere apologies to the groups' fans and give her account of the vicious virus.
She wrote: ''This started last week when we were meant to play [chat show] 'Conan'. And I woke up feeling really sick. I thought it was just a sore throat but didn't want to risk not being well for our own shows, so we cancelled.
''I stayed in and got some food and, of course, caught food poisoning from it... Because, why not? Not to get all graphic on you but it was a lot like that scene in 'The Exorcist' where she projectile pukes green and it's really violent.
''That ended up burning my throat which made it sore all over again. Then to make matters worse I woke up a couple of days later having gotten some sort of virus, which is what I'm still dealing with now. Clearly, it has not been my week.''
Hayley, 24, added she is hoping to be back up and running as soon as possible to avoid cancelling any more dates.
She said: ''I wish that my being sick didn't affect so many people and again, I am terribly sorry to everyone who I have let down by not being able to perform. Thanks to everyone who's sent me a get well soon message or any kind of support over the last week.''