Its not just the virus. A lot of his madness was due to 3 entities,
Dusk (The mastermind), Necromancer, & Paranoia (the one who
was mostly fucking with Eddie's mind).
With the Mercury-Virus powering him, he was actually going toe to toe
with Juggernaut. Not causing any damage, but he was actually able to
send him flying with punches, whereas before he could slightly affect
him, but nowhere close to the same extent.
After being transported to Dusk's realm of madness, Venom came
back with the Mercury Virus nearly dead and out of his system. He
literally broke in and just started punching Juggernaut, who literally
ran away saying, "I don't get paid to take this kind of abuse."
That last one was an especially embarrassing defeat for Cain to be
driven off by Venom.
Afterward, Eddie went back to the sewers and returned the virus
there.