The COVID-19 pandemic, which has afflicted the world for more than two years, is caused by SARS-CoV-2. The genome of this respiratory virus is composed of a single strand of RNA, a molecule similar to DNA that enables the virus to replicate in the body. Scientists from the Institut Pasteur, Inserm and the Paris Public Hospital Network (AP-HP) have developed a new method, CoronaFISH, to observe the virus and detect its presence.