Clarkson was one of several first-team players to be affected by the mystery bug and was sent home to recover at the end of August.
Having not started a first-team game since the 3-0 defeat to Cardiff, he only recently made his comeback in the reserves.
"It really hit those lads who had it for six," he told the Bristol Evening Post. "It was a very powerful viral infection and it swept through the club and hit us hard.
"The club tried to contain it by sending us home, but there was very little you could do about it once it was in your system.
"I was flat on my back and without an ounce of energy and I know it was the same for the likes of Lee Johnson, Lewin Nyatanga, Bradley Orr and Dean Gerken."