Lassa fever, a viral haemorrhagic fever with symptoms similar to those of Ebola virus disease, is endemic in much of West Africa and usually sparks a seasonal outbreak from December to March. Humans usually become infected with Lassa virus from exposure to urine and faeces of infected Mastomys rats. Lassa virus may also be spread between humans through direct contact with the blood, urine, faeces or other bodily secretions of a person infected with Lassa fever. These materials provide a general introduction to Lassa fever and are intended for personnel responding to outbreaks in complex emergencies or in settings where the basic environmental infrastructures have been damaged or destroyed.