The page for intestinal parasites says both that it requires 10 consecutive and a little later 10 cumulative treatments. At least on Stalker difficulty, I can attest to it being cumulative. I didn't see anywhere to make a mention of it other than just a comment and I don't have any sources other than cross-referencing the current page with my in-game expereince. I have had it twice recently and decided to see for sure by starting treatment, waiting several days, using another pill, and the counter went up. The counter never reset during the lapse.<ac_metadata title="Intestinal Parasites correction"> </ac_metadata>