Viruses invade a cell by latching onto certain proteins on its surface. Once attached, they can slip inside the cell and manipulate it into making new copies of themselves. But viruses cannot infect red blood cells. Unlike most other cells in the body, as red blood cells develop in bone marrow they lose their DNA. If a virus ends up inside a red blood cell, there are no genes it can hijack to replicate itself.

“It occurred to us that if a virus bound to a red blood cell, that was a dead end,” said Dr. Robert W. Finberg, a professor at the University of Massachusetts Medical School.
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To test the model, the scientists mixed normal bacteria with different levels of mutant traps and then infected them with viruses. After letting the viruses replicate, the scientists took a small sample to start a new colony. They discovered there was indeed a trap threshold above which the virus population could not survive. Above that threshold, the viruses disappeared by the time the scientists started the third round of colonies.