PC suffering from multiple diseases, mechanics

[Krik.longleaf]

So I have a pair of PCs who were subjected to Tetanus and failed their secret Fortitude saves. It's been untreated. And they recently contracted a faster-acting disease, and that night, they spent the time to Treat Disease, and the result would have succeeded against both diseases. So my question is: does the Treat Disease targeted at Monstrous Yeast affect the Tetanus? If so, how and when?
So the information I think is relevant:
from the Afflictions section of the rules "At the end of a stage's listed interval, you must attempt a new saving throw"
The listed interval for onset of Tetanus is 10 days.
And the wording for Treat Disease success is "You grant the creature a +2 circumstance bonus to its next saving throw against the disease."
So my initial thought is that the Treat Disease action, even though it wasn't targeted at the Tetanus, would still affect the next saving throw roll, which would take place at the end of the 10 day onset period. Does that sound right?

[YuriP]

No.The text of Treat Disease says "Attempt a Medicine check against the disease's DC"/"You grant the creature a +2 circumstance bonus to its next saving throw against the disease". My understand is that you choose the disease that you are treating once that "the" is not an "a". So you cannot target another unrelated disease with the same check.

[shroudb]

While I agree with YuriP that it should be a separate check, since they did beat the tetanus DC and basically spent 8 hours treating their patient, I would give them the knowledge at least (since the check was hidden, I'm not sure if you've spilled the beans to the party that they have tetanus) that they are indeed infected by it.

Something like "while treating them, you notice the early signs of a tetanus infection".