Sept. 11, 2011 -- They came from all walks of life. One was a fiery, athletic redhead. One was a stay-at-home mom and grandmother. Another was a buttoned-up businesswoman. They were all looking for love.
Between 2004 and 2007, they -- and several other women -- managed to cross paths with a man they describe as a serial killer. They say they were given a death sentence by 53-year-old Philippe Padieu, who knowingly infected them with the HIV virus.
In May, 2009, Padieu was convicted in a Texas court of aggravated assault with a deadly weapon for infecting the women. He was later sentenced to 45 years in prison.
The case of Padieu starts innocently enough. Diane Reeve, a martial arts instructor from Dallas, was braving the world of Internet dating. Encouraged by friends to get back on the dating scene after her marriage ended, she struggled to find Mr. Right until she met Padieu, a network security analyst from Frisco, Texas.
The two dated for four-and-a-half years. They were both martial arts enthusiasts, and Reeve eventually hired Padieu as an instructor at her studio. They took trips to New York, Paris and tropical islands -- they even planned to move in together. But in 2006, she discovered Padieu had been betraying her, she said, and they ended the relationship.
After the breakup, Reeve, who paid for Padieu's cell phone, perused his phone bill. She discovered numbers for nine other women whom he was apparently dating simultaneously. Some of the women had been, like Reeve, in extended relationships with him. But when she decided to call each of the women to warn them of Padieu's infidelities, she had no idea that these women would eventually become some of her best friends.
One of the people she called was a woman ABC News is calling Susan Brown. ("20/20" was asked to use pseudonyms for some of the women in order to protect their identity.) Brown had dated Padieu for over a year and, despite Reeve's warnings, decided to continue the relationship. But months later they separated, and Brown decided to get tested for sexually transmitted diseases, including HIV. Her doctor called her a few days later with the results.
"And she said, 'Well, I have some, I have some kind of bad news,'" Brown told "20/20." "'Your tests came out that you are positive for herpes.' And she said, 'But that is the least of our problems. You also tested positive for HIV.' And I just thought, 'I'm going to die, I'm going to die.'"
'What? You Gave Them My Name?'
Brown said she immediately called Padieu and told him he should get tested. But she didn't hear from him for weeks, she said. Finally, she called him again, and he said he had tested positive. Brown informed him that, at the health department's request for her to list any sexual partners, she had given his name.
"He said, 'What? You gave them my name? Why would you give them my name? You're trying to ruin my life. Why, why?'" said Brown. "And he was really upset with me, and I couldn't understand that. I thought now, this is kind of weird. He was really obsessed with the fact that his name was used. That made me very suspicious."
Based on the conversation with Padieu, she called the health department again and put them in touch with Reeve, who soon learned that not only had she contracted HIV, her immune system was severely compromised.
"I was at 31 [T-cells], I had AIDS," Reeve said.
But Reeve and Brown's shock and horror over their diagnosis was soon replaced with a mission to warn Padieu's other girlfriends. One of their first calls was to a woman ABC is calling Megan, a neighbor of Padieu's who had dated him for two years. She also tested positive.
The three women decided they needed help and contacted the Frisco Police Department to see if something else could be done. Cpl. Tom Presley took the initial call and complaint.
"It was clear to me that any intimate contact between Philippe Padieu and his victims was consensual," Presley told "20/20." "The difficult part that we had was to prove whether or not he knew that he had HIV when he had unprotected sex with them."
But while the police started to build their case, the three women decided to start their own investigation.
"We wanted to protect other women," Reeve told "20/20." "We didn't want it to happen to a single, solitary other individual."
The women say they had good reason to be worried about other victims. Megan, who still lived near Padieu, had seen women coming and going from his home. So, the women say, they took down the license plate numbers of the vehicles and had a friend track down the owners through the Department of Motor Vehicles. They called at least 23 women, some of whom were more receptive to the news about their boyfriend, Padieu, than others.
The women then took turns watching his house, followed women out of Padieu's driveway, even went through his garbage looking for possible names and numbers.
'He Knew He Was Getting Busted'
"I got one lady to pull over on the side of the road," Megan said. "She said I'm moving in with him,' and I said, 'you know, I'm HIV positive and I'm quite sure he is, too.' And she then moved her things out."
The police department, the prosecutor's office and the local health department knew what the amateur sleuths were up to and were very supportive.
"They were a remarkable group of women," said Presley. "They were feeding me with lots of information to make my life easier and to make the case successful. They were the ones that did the work."
While the police were trying to get more women to come forward, the county health department showed up at Padieu's house with an official document ordering him under Texas health code to "cease and desist any activity which puts others at risk of infection."
Padieu was warned about infecting others. But that did not seem to faze him, Reeve and Brown said.
"He was court-ordered," said Reeve, "and had a car in his driveway that night."
"And the night after," Brown chimed in.
But these desperate housewives were hot on his trail, and Padieu was beginning to feel the heat.
"He didn't know where it was coming from but he knew something was up," said Reeve. "He knew he was getting busted."
Watch the full story on "20/20" Sunday, 9 p.m. ET on "The Sixth Sense."