https://phys.org/news/2018-06-scientific-sleuthing-results.html

The beginnings of the discovery by the Eco-Systems Biology Group at the LCSB, led by Paul Wilmes, already go back some years: in their research, the scientists had found significant quantities of "small RNA" (sRNA) in human stool samples. This was, at first, an exciting result, since sRNA molecules play an important role in the body. sRNA molecules are involved in the regulation of gene expression, for example. Human sRNA molecules appear to be dysregulated in certain diseases, such as cancer.

The researchers took a look at these little molecules and compared them with known sRNA molecules from human blood. "We discovered that about half of the sRNA we isolated from human blood samples was not of human origin. They came from bacteria, fungi, foods and even mosquitos," Wilmes explains. "At the time, we thought that was an exciting result." The hypothesis at the time was that RNA from foreign organisms in the human gut makes its way into the bloodstream. If this turned out to be true, then it would be not only a fundamental expansion of our existing knowledge, but also a discovery with far-reaching practical implications: the molecules could be used as biomarkers for many diseases. A simple blood sample would give researchers and physicians an insight into the gut and the biological processes happening there.