Nadja Benaissa, of the German pop group No Angels, is in court accused of infecting a man with HIV when she knew she had the virus. Her defence is that she was "careless", but had been advised that the chances of passing on the virus were near zero. She was 17 at the time. It's not something that you can imagine happening to Girls Aloud, but I don't know why not – possibly Max Clifford would have intervened.

The charges are attempted aggravated assault and grievous bodily harm, and the verdict is expected on Saturday. In the UK she'd be prosecuted for reckless grievous bodily harm. These substantial differences between countries can mostly be put down to how effectively people campaigned on behalf of those with HIV, and how conservative the countries were to begin with. In America a tramp was given a 35-year sentence for spitting at a policeman, even though there is little evidence that HIV can be transmitted through saliva. In the mid-90s, when the virus was still thought of as a death sentence, Janette Pink successfully brought a case against Paul Georgiou in Cyprus. He was sentenced to 15 months, she went home "to die" (as reported in the British press), only to find that, even by 1997, the anti-retrovirals were surprisingly effective.

So that's the practical problem with criminalising the knowing transmission of the virus: it's no longer fatal. A typical person diagnosed in the UK will be in their mid-30s, in the early stages of infection, and can expect to live into their 70s. The handfuls of pills taken at hourly intervals have turned into one pill a day. HIV is a wonderful example of how fast and radically medical advances change lives when they go well.

Even if we disregard the fact that HIV isn't fatal, criminalisation is still incredibly bad law. It discourages people from taking a test, in effect forcing the disease underground where it can be neither monitored nor treated (three times as many new infections are caused by people who don't know their HIV status). Moreover, to show who gave it to whom is very difficult. The best use for phylogenetic testing is to show that a virus differs in strain, and so couldn't have gone from person A to person B. Showing that persons A and B have the same strain can't prove direction of travel, nor remove the possibility that they both contracted it from person C.

This is one of many reasons why intentionality is so difficult to prove. And yet in this country the judiciary relies on two sections of the ancient Offences Against the Person Act (1861) – Section 20, dealing with grievous bodily harm; and Section 18, grievous bodily harm with intent – to deal with transmission. But only the Section 20 charge of reckless transmission has ever stuck, intent being so tricky to prove.

There's a good argument anyway that the responsibility for safe sex is 50:50, and that making prophylaxis the sole concern, legally, of the person who's HIV-positive will foster irresponsible behaviour in everybody who believes themselves to be HIV-negative. This legislation won't reduce the spread of HIV, doesn't reflect the reality of HIV and doesn't deliver any meaningful justice, since there's no such thing as a "perpetrator" of Aids – everybody must have caught it from somebody.

Fleetingly, then, this looks accidental, an anomalous situation born out of a disease that threw everybody into a panic and then became manageable pretty swiftly. But why this health scare in particular? Why has nobody been held responsible for spreading flu, even in the worst days of the Spanish pandemic? Why is this the transmittable disease whose transmission equates to criminality?

Of course it's because it's sexually transmitted; this is about retaking the territory of sexual morality, from which the state has been systematically ejected. It is no longer acceptable to pillory the promiscuous merely for existing: if, however, their behaviour can be reframed as a concrete threat to others, rather than a nebulous menace to society, then authoritarianism is suddenly back in the conversation. The agenda of social conservatism – that promiscuity is wrong, that homosexuality is aberrant, that women should be the gatekeepers of sexual activity, since men can't help themselves (unless of course they're homosexual, in which case they should try harder) – is so thoroughly rejected in general terms that this opportunity to revivify it was too good to miss.

But if the disease isn't fatal, which it isn't, then we're dealing with an incurable but manageable condition. So, to keep the law in place, to make it consistent, to maintain its importance as a function of public health rather than sexual moralising, one would have to massively extend it: we'd have to make it illegal to pass on herpes or warts; make it illegal, for that matter, to snog someone when you know you have glandular fever; make it illegal to go swimming when you've got a verruca.

Alternatively, we could bin all this legislation, discreetly and globally, before anybody else winds up in court. Reports from the Benaissa trial have focused on what impact this episode will have on her reputation – but it's surely only the law surrounding the matter that is besmirched.