It’s the heyday of reality TV ... with fame-hungry hopefuls itching to hit our screens -- but one casting director tells us there’s no fast pass to fame if you can’t clear the ultimate requirement ... the STD test.

We caught up with Kristen Moss on TMZ Live Wednesday, and she broke it down -- if producers are tossing strangers in a house to explore each other sexually, they’ve gotta run heavy background checks ... and she says the big sensitive spot is STD testing, which wipes out a whole lotta hopefuls right off the bat! Gallery Launch Gallery

Yup, you heard that right -- plenty of wannabes are actually riddled with STDs, which means they’re axed from casting ASAP ... especially since these shows pretty much guarantee things are gonna get hot and heavy beneath the sheets.

While

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