After removal of all the red and white cells from blood, what's left over is plasma, a mixture of proteins and DNA fragments. These DNA fragments in plasma are remnants of the genomes of dead cells. Stanford scientists analyzed these fragments of so-called "cell-free DNA" to measure the prevalence of viruses in the body. They found that as drugs suppressed the immune system, the anellovirus proliferated. They think elevated levels of this virus could indicate a weakened immune system. Such an indicator could be a tool for medical treatment.