The Norwegian won his national time trial by over two minutes last week but then missed the road race championship on Sunday after picking up the illness.
His ride in the Tour de France was in doubt but he travelled to the Vendée region on Wednesday and has been confirmed in the final Team Sky starting nine that includes Bradley Wiggins, Simon Gerrans, Geraint Thomas and Juan Antonio Flecha.
"I never really felt sick; I just had a bit of a rash on my back but that's gone now and I feel just as good as I did before,” Boasson Hagen said in a statement from Team Sky.
"The doctor said I had to take it easy for a little while and get a bit of rest in order to recover as quickly as possible. I'm happy it's all worked out and I can focus on the Tour de France."
The 24-year-old, multi-talented rider is expected to play a key role helping Bradley Wiggins. He doesn't expect his enforced rest to make much difference to his form.
"I was feeling really good at the Dauphiné. I worked hard on my climbing there and it was nice to see that I could go in that front group and really help Bradley,” he said.
"Despite the rash, I haven't felt bad at any point since then so I'm really looking forward to the Tour where I'll be doing everything I can to help get a good result for the team, that's the main thing."
Team Sky will target victory on the first stage to Mont des Alouettes with Ben Swift and Boasson Hagen and then hope to put a rider in the yellow jersey with success in Sunday’s second stage team time trial around Les Essarts.