Every day, hundreds of thousands — if not millions — of molecular reactions are happening in laboratories worldwide. Small droplets of liquid that give us a lens into an individual’s respiratory pathways are analyzed for whether or not they contain the pathogen of the year: SARS-CoV-2, the virus that causes Covid-19. The technique used for this analysis is called polymerase chain reaction (PCR), and it exploits the ability of genetic material to replicate. Although imperfect, it’s been critical in diagnosing the disease by amplifying genes specific to SARS-CoV-2. When it’s accurate, PCR helps confirm positive cases, slows the spread of infection, and allows health officials to treat individuals who have Covid-19.