HIV may have been passed between two sisters who shared a razor to shave their legs, scientists said today.
A shared razor may have spread HIV
The report, in the journal Aids, is described as a "sobering" reminder that the disease can be spread in unusual ways.
In this case, an 18-year-old girl from Australia caught the disease on the first occasion she had sex.
Her 16-year-old sister later tested positive for the virus when she donated blood.
Both teenagers had a particular subtype of the virus which is very rare in Australia, making it highly likely that one had infected the other.
The older girl was unaware of her HIV infection until doctors began to investigate how her younger sister - who had never had sex - had contracted the virus.
Extensive questioning by doctors revealed only that the pair shared a bathroom, and perhaps occasionally had used the same razor to remove body hair.
HIV can be spread through contact with blood or other body fluids from an infected person, so it is possible that the 18-year-old nicked her leg with the razor, left blood on the blade, which was then passed into a similar cut by 16-year-old.
However, the experts who assessed the case say that findings do not represent a "significant risk factor" for HIV spread.
It remains primarily a disease spread either by sex, or by the use of contaminated medical equipment, perhaps as part of injecting drug use.
Early fears that the virus could be transmitted through kissing, hugging or even sharing the same lavatory seat have been quashed.
There have, however, been other cases in which HIV has been passed with no obvious explanation.
In one case - also in Australia - a mother may have contracted HIV after applying ointment to psoriasis scales on the skin of her infected son.