I was 50 and had been divorced for a year and a half when friends encouraged me to try to meet someone new. I wasn't keen on the idea of bars or other obvious places, so I decided to brave the world of online dating.

After posting my profile on several sites, without much success, along came Philippe Padieu, a 46-year-old network security analyst. First impressions were good. He was handsome and charming, and after a few phone calls and emails, we arranged to meet.

Although most of the conversation centred on him, the date went well and I liked him. We began seeing each other regularly.

On our first date, I asked Philippe if we were seeing only each other – if not, he should use a condom. Although he seemed annoyed by this question, he assured me there was no one else, and that's when our relationship became more serious. He was always there for me when I needed him and we were, I thought, incredibly close. I trusted Philippe and never believed he would do anything to hurt me. Over our four and a half years together, we travelled the world and shared a life. I hired him as a part-time instructor at my martial arts studio – he ran t'ai chi classes and I taught taekwondo. I was in love and ready for a long-term commitment. But several months before we were due to move in together, I noticed changes in our relationship. Everything had become a bit routine and Philippe seemed strangely distant.

One night after dinner with friends, I stopped at my studio and noticed Philippe's car parked outside. Thinking that was a bit odd, I went in. The door to the private lesson room was shut. As I called his name, Philippe opened the door – he was with one of his young students. She went white; I was furious. Philippe begged for forgiveness, and told me he loved me and that he'd slipped up. Stupidly, I believed him.

Months later, he did it again. We were at my daughter's wedding, on our way to the reception, when Philippe told me he needed to go home as he wasn't feeling well. Later that evening, I went to his house to see if he was OK. He wasn't there. Instinctively I knew he was with another woman. Sitting in my car on his driveway, I started to cry – I felt betrayed, then angry. I called the phone company and asked for the pass code to his mobile – I was paying his account so it was in my name anyway. I checked his voicemail and my fears were confirmed. Two women had left messages – both confirming dates with him. At that moment, his car came round the corner. He got out and I screamed at him; he screamed at me, bashing my car with his fist. That's when I knew it was over.

Two days after we broke up, I went for a routine medical checkup and less than a week later I was told I had a sexually transmitted disease. I was devastated but determined to warn others and checked all Philippe's mobile phone records. I discovered he'd been dating nine other women. Most of the women Philippe targeted were lonely and tired of the dating scene.

One of his girlfriends, Susan, whom he had been seeing for a year and a half, followed my advice and got tested for STDs. But the news was much worse: she had tested positive for HIV. Soon after I found out I had Aids. Two weeks later, I told the police.

While they began investigating, Susan and I took matters into our own hands, setting up surveillance across the road from his house. We took down numberplates and tracked down more than 23 women to warn them. Twelve had HIV.

I filed charges against Philippe in 2007 and two years later, his case came to trial. In court, 10 of us – all HIV positive – testified against him. Philippe was found guilty of six counts of aggravated assault using a deadly weapon – the deadly weapon being his bodily fluid. The court was able to prove Philippe knew about his condition in 2005 and continued to have unprotected sex. I'll never know his exact motivation, perhaps a deep-seated anger towards women? He was given 45 years and won't be eligible for parole until he is 75. For now my health is stable; I feel less anger but I can never forgive him. He changed our lives for ever. On the day he was sentenced, we all held hands and cried, vowing never to be so trusting again.

• As told to Nicole Partridge.

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Susan's name is a pseudonym