Although Fletcher did mount a comeback bid after originally succumbing to the virus in March, he was so obviously underweight when he featured in the season-ending Premier League encounter with Blackpool that Sir Alex Ferguson quickly abandoned plans to press his fellow Scot into service at the following week's Champions League final.
Even when Fletcher returned for pre-season training he still had not regained all the kilos lost.
So, instead of heading off to the United States with the rest of his team-mates for a well-received three-week tour, the Scotland skipper remained at Carrington to bulk up, still unsure exactly why he was in that state to start with.
"It has all been resolved now but I never got to the bottom of it," he admitted.
"I felt drained and weak and was bed-bound for a bit.
"But the biggest thing was losing the weight.
"The virus was bad but for someone like me to lose the weight I did, it was really difficult to put back on.
"It took me a good few months to get back to my natural weight. "Eventually I got there though and I feel stronger for it now.
"I have even put on a little bit more, which will be beneficial for me as well."
Aside from a 17-minute substitute appearance in the second leg of the Champions League semi-final against Schalke, Fletcher played just once from March 6, until the beginning of this month, when he played back-to-back Euro 2012 qualifiers for Scotland.
Filling the space in between has not been easy.
"I felt fine in the Blackpool game but I needed a lot of meal replacement drinks and protein, in addition to my normal diet.
"I did a lot of work in the weights room as well.
"I always stayed positive. With the manager, doctor and physios I knew I was in good hands.
"I was always confident I would be back."
At the end of last season, having missed out on the chance of playing in a Champions League final for the third time, Fletcher must have expected the biggest danger to his place in Ferguson's preferred starting line-up would come from a new arrival.
Wesley Sneijder, Luka Modric and Samir Nasri were all touted.
Instead, Tom Cleverley, back from a successful loan spell at Wigan, stepped into the breach, alongside Anderson, who finally started to show why Ferguson paid big money to sign him from FC Porto four years ago.
"The young lads have been fantastic," said Fletcher.
"They have gained experience at different clubs so it is not as if they are new to Premier League football, but they have been like a breath of fresh air.
"The results speak for themselves. Long may it continue.
"It is going to be difficult for me to get back in the team.
"That is the challenge in front of me. I am looking forward to it."
Indeed, Fletcher intends to be around if his younger team-mates do have a dip in form, which the 27-year-old hopes does not happen.
He certainly does not intend to allow them to get carried away by all the plaudits they have been receiving, praise which he admits may have irritated Chelsea, who head to Old Trafford on Sunday.
Champions the season before last, the Blues are almost the forgotten men of this season, which has so far been dominated by Arsenal's troubles and the brilliance provided by both halves of Manchester.
"I think so," said Fletcher as he pondered whether a team so used to challenging for top honours could be piqued because they are being ignored.
"They started last season on fire and ultimately did not win the league, so you have to be careful.
"Chelsea are a top side, with a lot of experience. They have a new manager and have brought in some younger, fresher, exciting players.
"It is a tough Premier League. Although the two Manchester clubs have started particularly well, the other teams are going to be dangerous."