Genital herpes contracted as a result of a girlfriend’s fling compelled a Manhattan man to seek revenge on her former lover by luring him to Suffolk County and shooting him to death in January 2009, a Suffolk prosecutor told jurors Thursday.

But the attorney for Daniel Greenspan, 30, said the only evidence that his client ever set foot in Suffolk — let alone killed Michael Sinclair, 32, of Brooklyn — comes from the lying mouth of his then-girlfriend, who may serve no time in prison despite once being charged in the case with murder herself.

Greenspan is on trial in Riverhead, charged with second-degree murder, before state Supreme Court Justice William Condon. Another man, David Belton, 29, of the Bronx, has pleaded guilty to second-degree attempted murder after two trials ended in hung juries.

During Belton’s trials, prosecutors argued that the three lured Sinclair to Kellum Street in West Babylon with the intent of robbing him. But in her opening statement Thursday, Assistant District Attorney Nancy Clifford presented a completely different theory of the case.

“It was intentional and it was personal,” Clifford said of Greenspan’s reason for shooting Sinclair in the chest and head. “While they didn’t know each other, they had something in common — a young woman named Noriella Santos.”

Santos, 28, had dated and occasionally lived with Greenspan in the Bronx since they were in high school, Clifford said.

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Greenspan, then known as Daniel Kraeger, later legally changed his last name to Rivera, Clifford said. He later legally changed it back to Kraeger and then to Greenspan, and lived for a time in Israel, she said.

In summer 2008, they were apart and Santos slept with numerous men, including Sinclair, Clifford said. That September, she was diagnosed with genital herpes.

She then got back together with Greenspan and gave the disease to him, Clifford said. Santos told Greenspan it came from Sinclair, even though she didn’t know who it came from, Clifford said.

Greenspan became obsessed with Sinclair and persuaded Santos to set up a confrontation in a place unfamiliar to Sinclair. She invited him to a nonexistent party in West Babylon on Jan. 31, 2009, Clifford said.

“She never pictured it ending in Michael’s death,” Clifford said. But after Belton initiated a robbery in the street, Clifford said Greenspan took the gun from Belton’s shaking hand and shot Sinclair.

The killers left Sinclair’s phone behind, and it showed calls with Santos, Clifford said. Her phone records had calls with Belton and Greenspan, Clifford said.

Defense attorney Arthur Aidala of Brooklyn said the entire case against his client rested on nothing but Santos.

“How does she get out of it?” Aidala said to jurors. “She sets up Michael Sinclair and leads him to his death.”

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She got out of murder charges by blaming Greenspan and pleading guilty to attempted robbery, Aidala said.

“She’s a master manipulator,” he said. “A master deceiver. Detectives talk to her. She admits what she can’t deny. She denies what she can’t admit.”

So instead of spending decades in prison, she now faces just a few years at most, he said.

Jurors should also consider her actions after Sinclair was shot. If she wasn’t involved, Aidala said, she might have screamed or banged on neighbors’ doors, or called 911. But instead, he said, Santos just left.

There is no physical, forensic or even circumstantial evidence that Greenspan was ever in Suffolk County other than Santos, Aidala said. “It’s only her,” he said. “That’s it. There’s nothing else.”

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CORRECTION: An earlier version of this story misstated the charge to which David Belton pleaded guilty in the death of Michael Sinclair.