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Doctors are rushing Alex Trebek in for an unexpected round of chemotherapy ... because his battle with pancreatic cancer has taken a turn for the worse.

The beloved "Jeopardy!" host announced Tuesday morning, "This past summer we thought I was finished with chemo. That was a bit premature and certainly overly optimistic." He says he started immunotherapy ... but it didn't work.

As Alex put it, under immunotherapy ... his "numbers went south, dramatically and quickly."

He says he's going back in for another round of chemo, and as he's done ever since announcing his diagnosis ... he's keeping his amazingly positive outlook.

Alex smiled as he said the chemo "worked very well the first time! So, we're expecting good results again."

As TMZ first reported, Alex was diagnosed with stage 4 pancreatic cancer back in March. From the get-go he made it clear his intent was to finish recording that season of "Jeopardy!" ... and also to return for the current season.