Mobile detection of Lassa virus Lassa fever is a hemorrhagic viral disease endemic to West Africa. Usually, each year sees only a smattering of cases reported, but hospitalized patients risk a 15% chance of death. Responding to fears that a 10-fold surge in cases in Nigeria in 2018 signaled an incipient outbreak, Kafetzopoulou et al. performed metagenomic nanopore sequencing directly from samples from 120 patients (see the Perspective by Bhadelia). Results showed no strong evidence of a new strain emerging nor of person-to-person transmission; rather, rodent contamination was the main source. To prevent future escalation of this disease, we need to understand what triggers the irruption of rodents into human dwellings. Science , this issue p. 74 ; see also p. 30