
Speaking for the first time since his return to the team against Oxford last weekend, Zetterstrom said it was good to be back after he spent three months out injured.
Zetterstrom picked up a virus, the aftermath of which left him unable to play or train with head coach John Eustace admitting the other week there was a point that he felt he might not feature again this season.
Asked whether he could explain more about why he was out for such a long time, Zetterstrom said: "I had an infection that gave me some complications that made me unable to play.
"That's all behind me and I feel miles better.
"I am happy to be back and feeling better as well.
"It's very good to be back and I am feeling very good at the moment."
Zetterstrom said that Derby's staff had helped him considerably during his time out with the Sweden international now targeting a place in their World Cup squad.
The goalkeeper said that he was keen not to rush his comeback and added: "You have to take every day as it comes.
"It's so easy to look forward and make things even bigger than what they might be.
"We were in a very good spot to make the play-offs, there was a World Cup ahead.
"You just have to relax and not think about those things.
"If you look ahead too much, that's when your head sort of goes.
"The medical staff and the coaching staff have all been there for me.
"I can't stress enough how grateful I am for that.
"Whether it was taking care of me or giving me a call, just a little text, it's been fantastic. It meant the world."