In January 1969 an unknown virus was isolated for the first time from the sera of two nurses, who died.… The infection, being called Lassa fever, involved almost all the body’s organs.… Doctors so far suspect that the disease was transmitted by an animal, but what animal is not known. It is also believed that the patients can acquire the infection from one another, but only through more than casual contact.
Update
Named for the Nigerian village where cases first appeared, the Lassa virus causes hemorrhagic fever and kills about 5,000 of the hundreds of thousands of people infected each year in West Africa. The virus, spread by the Natal multimammate rat (Mastomys natalensis), can be transmitted through human body fluids.