Ok, I did some further investigating on this matter. You can still gain off poisoning with Infectious Strike. However, at 50 poisoning I used player made potions (didn't seem to gain off NPC ones, maybe it was a fluke but I doubt it). I consistently gained up to 67 skill using Infectious Strike with a poison potion. After that, the skill gains slowed down drastically. I then switched over to just poisoning with poison potions (not using Infectious Strike) from then on and have enjoyed good success thus far.
So in the end, you can bring up your skill fairly high with Infectious Strike and a poison potion (not greater) until 67. Just thought I'd share my experience.
Hendric