Lassa fever and Marburg virus disease.

In the light of progress in virology, it comes as a surprise to discover a disease, an epidemic disease moreover, caused by a "new "virus. This has happened twice in recent years. The diseases, Marburg virus disease and Lassa fever, are caused by unrelated viruses that are highly pathogenic for man and both have been responsible for illness and death among laboratory scientists and medical personnel. Both diseases are endemic in the continent of Africa and are known or suspected to have natural transmission cycles in nonhuman vertebrate hosts. This article, by the Director of the WHO Regional Reference Centre for Arboviruses, briefly describes the historical background, the epidemiology, and the clinical manifestations of these diseases, which are of interest to WHO because of their appearance among health personnel and because they can be mistaken for yellow fever.