Lassa virus budding. Coloured scanning electron micrograph (SEM) showing lassa virus (Lassa mammarenavirus, LASV) particles (virions) budding off a Vero culture cell. L. mammarenavirus is an arenavirus that causes Lassa haemorrhagic fever, a form of viral haemorrhagic fever (VHF), in humans and other primates. A surge in Lassa fever cases in Nigeria in 2018 does not appear to be linked to a single virus strain or increased human-to-human transmission, according to a genomic analysis published in The New England Journal of Medicine.