As the world has learned to its cost, the Delta variant of the pandemic coronavirus is more than twice as infectious as previous strains. Just what drives Delta’s ability to spread so rapidly hasn’t been clear, however. Now, a new lab strategy that makes it possible to quickly and safely study the effects of mutations in SARS-CoV-2 variants has delivered one answer: a little-noticed mutation in Delta that allows the virus to stuff more of its genetic code into host cells, thus boosting the chances that each infected cell will spread the virus to another cell.