How do you prove it?

This is not necessarily the case. Sometimes, the person who complains to the police after recently testing HIV positive turns out to have more likely infected the person they are accusing, so it is important to be sure before making any allegation.

It is also possible that the person making the complaint (‘the complainant’) was infected by someone else, or in another way (for example via sharing drug injecting equipment or a blood transfusion in a foreign country) months or even years before they met the person they are accusing. . With something like herpes, many people carry the virus without ever knowing it.

If your sexual history or testing history shows that you have had sex with a number of other partners, it is likely that they will all have to be eliminated before a case can proceed. The police should make all these investigations before referring the case to prosecutors. The Crown Prosecution Service (CPS) for England and Wales will not bring a case where this work has not been done by police.

Complex scientific tests – known as 'phylogenetic analysis' – should always be done in HIV cases to compare the viruses of the complainant and the accused. If the two viruses are different then this proves that there was no HIV transmission between the two people.

If the viruses appear to be similar this shows that HIV transmission from the accused to the complainant could have taken place, but it does not prove it ‘beyond reasonable doubt’. It is still possible the complainant in fact transmitted HIV to the accused, or that both were infected by people sharing the same type of virus – or by the same person. This is particularly likely where people within the same social circles have all gone out with each other.