Lassa fever is endemic to West Africa. An international team of researchers has developed the largest genomic data set in the world on Lassa virus (LASV). The new genomic catalog contains nearly 200 viral genomes collected from patient samples in Sierra Leone and Nigeria, as well as field samples from the major animal reservoir, or host, of Lassa virus--the rodent Mastomys natalensis, also called the multimammate rat. The researchers show that LASV strains cluster into four major groups based on geographic location, with three in Nigeria and one in Sierra Leone, Guinea and Liberia. Although Lassa fever was first described in modern-day Nigeria in 1969, the current study also suggests that these four LASV strains originated from a common ancestral virus more than 1,000 years ago and spread across West Africa within the last several hundred years.