People often make assumptions about who infected them with HIV or a sexually transmitted infection (STI). This is usually based on the idea that the person who tested positive first was the person infected first.

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

Is it possible to know who passed on the virus?

It is possible that the person making the complaint (‘the complainant’) was infected either by the person they’re accusing or by someone else. They might also have been infected in another way (for example, when sharing drug injecting equipment or during 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 as suspects 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. 

Can the police prove who passed on the virus?

Complex scientific tests called 'phylogenetic analysis' should always be carried out 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 it means 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 that it was in fact the complainant who had transmitted HIV to the accused, or that both were infected by the same third person, or by different people sharing the same type of virus.

This is particularly likely in cases where people within the same social circles have all had sex with each other.

If you need help with matters concerning HIV transmission, call our free helpline THT Direct.