Researchers have simulated SARS-CoV-2 infection in human lungs, thereby generating key insights into the mechanisms involved. Using cultured lung tissue samples, the researchers showed that the virus responsible for COVID-19 has only limited capacity for directly infecting cells within human alveoli. The majority of viruses which reach the lungs are ingested by macrophages (cells of the innate immune system). Ingestion activates a targeted anti-viral immune response within these immune cells.