The case captivated the world in 2007, and it remains a mystery today.

Blonde, blue-eyed Madeleine McCann disappeared from the Portuguese resort where her family was staying, just days shy of her fourth birthday.

As money poured into a worldwide "Find Madeleine Fund," her parents repeatedly took to the airwaves to plead with whoever abducted their daughter to bring Maddy back. Kate and Gerry McCann were the picture of desperation. But to one con man, they were an easy mark.

Kevin Halligen had already carefully crafted the image of an international man of mystery by 2007. An affable Brit with the gift of gab and the ability to win over a pitch meeting with ease, Halligen had somehow managed to parlay a job at a battery factory that did some defense work in England into a career as a global security consultant.

But a former employee, Henry Exton — who really was once a British spy — said in fact, Halligen was living in a carefully constructed fantasy.

"He picks parts of people's personalities and adopts them as his own," Exton told "American Greed." "Stories of what they've done, how they've behaved and why they've done certain things. And they become his stories."

Whatever Halligen's real story, his reputation was enough to win a six-month contract worth nearly a million dollars from the Find Madeleine Fund. Henry Exton says Halligen knew exactly what the McCanns wanted to hear.

"The promises were varied," Exton said. "But they included that he would have the ability to identify who had used phones on that evening and get satellite imagery of people walking around that night. And it was phenomenal stuff."

Phenomenal perhaps, but also fiction. In the end, Exton said, all Halligen managed to produce was a printout of a Google map. But all the while, he was spending tens of thousands of dollars on a chauffeur, fancy meals and hotel rooms around the world.

Kevin Halligen was using one of the oldest tricks in the con man's book: preying on the desperate. And the annals of "American Greed" are lined with equally despicable cases.

Here are some of the more infuriating examples.

Kansas City pharmacist Robert Courtney claimed he was doing it to pay off a $1 million pledge to his church. But his scam uncovered in 2001 surely violated multiple commandments.

The son of a preacher, Courtney was active in his church. And after buying two pharmacies in the 1980s, he became a prominent member of the community. Cancer survivor Liane Dillman recalled how Courtney would sometimes personally deliver her prescriptions.

"He'd say, 'Oh, you don't need to drive all the way to the store,' because his pharmacy was across town where my doctors were. So he would volunteer to bring them," she told "American Greed."

But Courtney was also carrying with him a dark secret. He eventually confessed to federal agents that he had been diluting chemotherapy drugs for nearly a decade, compromising vital treatment for thousands of cancer patients.

Robert Courtney is serving a 30-year sentence at a federal prison in West Virginia, after pleading guilty in 2002 to 20 felony counts. At his sentencing later that year, he claimed remorse.

"It's hard to find the words to use because the words seem so inadequate," he told a packed courtroom. "I have committed a terrible crime and I deeply and sincerely regret it. I wish I could change everything. I don't know why I did this. I didn't need to. I'm very sorry that I did."

Many of his patients, including all eight named in his criminal case, have since died.

Watch "Deadly Rx for Greed" on Hulu.

It was the miracle diet drug of the 1990s, and the manufacturers of Fen Phen marketed the product to people who had tried everything else to lose weight but just couldn't. But patients soon learned to their horror that fen-phen was linked to a potentially deadly and irreversible heart condition. Now, instead of just worrying about their weight, the patients legitimately feared for their lives.

"I couldn't be a wife. I couldn't be a mother," Connie Sue Centers told "American Greed." "I was working as a nurse. And I worked as long as I could and I just had to quit 'cause I was so short of breath."

American Home Products paid out billions of dollars in nationwide class action settlements over the drug. Meanwhile, a team of lawyers in Kentucky saw those settlements as a chance for their own piece of the action.

Attorneys Shirley Cunningham, William Gallion and Melbourne Mills recruited 440 of the most desperate fen-phen patients — including Connie Sue Centers, who recalled how the attorneys won her trust.

"When you don't know anything about any money, you are going to trust this person that's helping you — supposed to be helping you — because I didn't know anything about it."

The team eventually won a $200 million settlement with American Home Products. But rather than distribute the money to their clients to pay for badly needed medical care, the lawyers kept most of the $200 million for themselves, buying expensive homes, a thoroughbred race horse and making lavish donations to the University of Kentucky in exchange for courtside seats at UK basketball games.

"They were out buying cars with our money before we ever knew that we were getting anything," Connie Sue Centers recalled. "And me still getting food stamps and hanging sheets to my windows. You know, it was just ridiculous. It still is ridiculous. The more I talk about it, the madder I get."

After two trials — the first ending in a hung jury — Cunningham and Gallion were convicted in 2009 of fraud and conspiracy, ordered to pay $127 million in restitution and another $30 million in criminal forfeiture. Both remain in federal prison.

Melbourne Mills, who had been the public face of the campaign to recruit clients, was acquitted after raising alcoholism as a defense. But Mills was eventually disbarred.

Victim Jacquelyn McMurtry told "American Greed" that the outcome of the case was cold comfort at best.

"There's no closure," she said, "and there never will be even if I get all the money back. It's not just that they stole from us. There's more to it than that. I just felt like they didn't care if I died or not."

Anyone who has suffered from sinus problems knows how desperate the search for relief can be. And it's not just the pain and discomfort. In the most serious cases, sinusitis can lead to blood clots and vision problems. Sinus infections can even spread to the brain.

But an Indiana surgeon saw one thing in particular up his patients' noses: money.

Mark Weinberger set up his sinus surgery center in Merrillville, Indiana, in the heart of the Rust Belt. The concentration of steel mills and factories in Northwest Indiana meant there were lots of people with breathing problems — and generous union health insurance plans.

Bill Boyer was a heavy equipment operator by day and a bouncer by night.

"I was an amateur boxer back in the '70s, and my nose was broken several times," he told "American Greed." "I was having trouble with breathing through it, snoring, and I just decided to see if there was anything to be done to get it fixed."

He had seen Dr. Weinberger's billboards along the highway, and finally decided in 2003 to go in and see him.

Boyer said the doctor performed a CT scan.

"He points to the computer there and shows me these really terrible images on there saying that was my sinuses," Boyer recalled. "I asked him, I go, 'That's in me?' and he said, 'Yes.'"

The images included polyps and a deviated septum. But fortunately, Weinberger said, he had the answer: endoscopic sinus surgery, which he performed on Boyer a few weeks later, billing Boyer's insurance company for $42,000.

But the surgery brought no relief, in part because Weinberger had never performed it. Another doctor later determined that all Weinberger had done was to bore two holes in Boyer's skull. And that's not all. Those troubling images from the CT scan? They were someone else's. Boyer's sinuses, it turned out, were essentially normal.

It turned out Bill Boyer was one of hundreds of Weinberger patients taken in by the same scam. In at least one case, the consequences went far beyond delayed relief.

Phyllis Barnes had gone to see Weinberger in 2001 complaining of hoarseness and weight loss, and reporting she had been coughing up blood. Weinberger's diagnosis? Nasal polyps and a deviated septum, which endoscopic sinus surgery would take care of. But in fact, it was later determined Phyllis Barnes had throat cancer, which eventually killed her.

"His diagnosis was never for the welfare of his patients, Barnes family attorney Ken Allen told "American Greed." "His diagnosis was a diagnosis for dollars, and for Phyllis that was a death sentence."

Weinberger grew rich off his patients' desperation — private jets, yachts and a $2.4 million mansion on Chicago's Gold Coast, from which a chauffeur-driven limo would take him the 45 miles each way to and from Merrillville.

But as the malpractice claims began piling up in 2004, Weinberger would take a much different journey.

On vacation with his wife in Greece, he disappeared. It would be another five years before authorities would catch up to him, following a failed suicide attempt in the Italian Alps.

In 2012, after Weinberger plead guilty to 22 counts of health care fraud, a federal judge sentenced him to seven years in prison. But U.S. Bureau of Prisons records show Weinberger was released earlier this year. Separately, one civil jury ordered Weinberger to pay Bill Boyer $300,000 for malpractice. Another jury awarded $13 million to Phyllis Barnes' estate.

Watch "Mark Weinberger: Nose No Bounds" on Hulu.

Is there a moral to all of these stories pitting greed against desperation?

For one thing, greed will always win if you let your desperation get the better of you. Resist the instinct to instantly embrace anyone who offers to help, especially if others have already given you a less hopeful response.

Desperate times don't just call for desperate measures. They call for you to keep your wits about you and keep your guard up. Train yourself to think that way now, so that if you do find yourself in trouble, you won't fall victim to the worst kind of fraudster.

Watch "American Greed," Thursdays at 10p ET/PT on CNBC Prime.