The 'Oh Love' band cancelled a concert in Bologna earlier this week when Armstrong was rushed to the hospital. He was later released after several hours under doctors' care.
Speaking to MTV News, Green Day bassist Dirnt admitted he was initially very worried about Armstrong's health.
"Billie came down with heavy, heavy dehydration; it was like severe dehydration, influenza, and it was just a really, really bad situation," Dirnt explained.
He went on to say: "He woke up in the morning and it was bad. Let's put it this way, if you're in your hotel room and you're vomiting profusely, it's not fun. So what are you gonna do? It's hard to sing while you're puking."
Armstrong has spent the last few days recovering, but will perform with Green Day at the MTV Video Music Awards later this evening.
Dirnt promised that not even Armstrong's illness will prevent Green Day from being at top form on the awards show.
"We're ready to rock, we're gonna be here, we're gonna have a good time… We're going to drink chicken blood," he joked.
One Direction, Taylor Swift, Nicki Minaj, Rihanna, Frank Ocean, Pink and Alicia Keys are also performing on the VMAs.
The 2012 MTV Video Music Awards air from 8pm to 10pm ET tonight (September 6).
Watch Green Day's 'Oh Love' music video below: