Mecha anime with a "mind control virus"
Your anime might be: Mobile Fighter G Gundam



Firstly it was aired in Toonami in 2002, so it fits the initial bill.

Then in the wikipedia page, The plot section has this one specific fragment:


  As they encounter Gundam pilots who had come in contact with the Dark
  Gundam, Domon and Rain learn of its unique cellular properties to
  regenerate, multiply, and evolve by infecting organic matter and
  causing violent behavior in living things.


So we know that this anime is about mechas and there is a virus thing, namely the Dark Gundam, that has abilities to corrupt organic matter and evolve in/with it (or something along those lines).

Then as I searched for footage of the anime, I came upon this video of the first episode which in its first seconds shows the character going through a circle in order to wear a suit, just like you also told.

I'm not sure about the meeting part, for I haven't watched it, but as for everything else you've told, it's a dead giveaway.