The band was forced to axe an appearance on Conan O'Brien's U.S. chat show last Tuesday (30Apr13) when the singer fell ill with a sore throat, and Williams subsequently battled food poisoning.
As a result of the health concerns, the group called off its planned gig in Salt Lake City, and Williams has taken to her page on Tumblr.com to apologise to fans and open up about her illnesses.
She writes, "This started last week when we were meant to play 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.
"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 tonight. Thanks to everyone who's sent me a get well soon message or any kind of support over the last week."