For someone living with HIV, the number on a blood test can run their whole week. The drugs are supposed to push the virus below the threshold a machine can see, and most of the time they do. So when the lab comes back with a reading above 20 copies per millilitre, the worry sets in. Has the treatment failed? Is the virus loose again, ready to infect a partner? For a small group of patients, that reading keeps coming back month after month, year after year, with no obvious reason. Clinicians...