What if some of your cells were not your own? What if they once belonged to someone else?

Part mind-bending medical mystery—part cutting-edge science—Hidden Guests uncovers the astonishing phenomenon of microchimerism: the presence of foreign cells inside our own bodies. The incredible story of how those cells got there—and what they do once they arrive—might change everything we know about the immune system, lineage, and identity.

We are all told the same story as children: that we grew from a single cell into a human, that all of our cells came from the first fertilized egg, and that we have one distinct genetic code. But scientists are beginning to challenge that story.

The discovery of microchimerism shows that not all our cells are our own—some of them migrated from other bodies. How did they get there? Scientists are still studying their journey, but today we know cells are exchanged in pregnancy, through transplants and blood transfusions, and possibly even through sex. But what does this mean for our daily lives—is it really such a big deal if someone else’s cell turns up in our bodies? The answer is, as author Lise Barnéoud shows in Hidden Guests, that the implications could be earth-shattering.

In Hidden Guests, Barnéoud interviews doctors, researchers, and medical experts at the forefront of microchimerism research. She interweaves their fascinating discoveries with the shocking human stories of microchimerism including:

The story of the mother who gave birth to the genetic children of her sister … even though her sister had never been born.

The story of the man whose DNA was found at a crime scene—only he was in prison at the time. It turned out that he had received a bone marrow transplant, and the DNA came from his donor—the actual offender.

The story of a cancer survivor who discovered that the cells in his blood, saliva, hair, and even his semen were slowly being replaced by the cells of his organ donor

The story of a woman whose children were nearly taken away after genetic testing showed she was not their mother—until she proved that their DNA came from a vanished twin whose cells she had absorbed in utero

Hidden Guests traces the history of this still-emerging science while asking philosophical and probing questions about immunity, biology, evolution, parental testing, criminal forensics, and the concept of individual identity. Barnéoud makes the case for expanding our notions of both self and immunity: as ever-changing collectives of cells in relation, we are not unlike ecosystems. And like ecosystems, perhaps, the greater our diversity, the greater our resilience.