The biological components of an individual drone could be infected with an organic virus, but the Borg nanoprobes in their bloodstream were capable of seeking out and destroying infected cells, as mentioned in the Voyager episode Someone to Watch Over Me. You are correct about aliens infecting the Borg with a virus, which happened in the episode Infinite Regress, but it was a synthetic one designed to infect their computer systems rather than the organic components. The Federation also designed a similar tactic in the episode I Borg, designing a program to disrupt the collective, but again did not seem to even consider an attack on the biological components of the Borg.
It's also worth noting that the Borg collective has no qualms about sacrificing individual drones, so any ship infected with a disease too virulent for their nanoprobes to destroy would likely be purged completely before it had a chance to spread the disease as long as there was a way to detect it.