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Lassa fever is a contagious viral disease that can be passed from animals to humans and from humans to humans.  The virus that causes the disease is passed in the stool and urine of infected persons.  It is difficult to diagnose because most of its symptoms are non-specific to the disease.  However, it is characterized as a hemhorragic fever as it causes bleeding of the mucous membranes.

Lassa fever was first fully described in 1969 during an outbreak in Lassa, Nigeria.  It is known to be endemic to most countries in west central Africa.  It's primary vector in the wild is the Natal Multimammate mouse, which is known to raid grain stores and is often consumed as well, thus spreading the disease.  

Luckily, in 80% of cases, the virus causes no symptoms or mild symptoms.  However, in the other 20% of cases, the disease can be life threatening.  When patients are treated quickly and aggressively with Ribovirin, the mortality rate is low.  However, the disease can be particularly dangerous to pregnant women, often requiring that the fetus be aborted  in order to preserve the life of the mother.  In addition, most hospitals where the disease is endemic are ill suited to diagnose and treat the disease, and about 15-20% of patients who are hospitalized for it will die.  

Diagnosis of the disease is difficult.  Most of the symptoms are prosaic.  Blood tests can be useful as patients will generally show an atypically low white blood cell count for a person with a fever, as well as a low platelet count and a high AST level.  Patient's can be tested for the antibodies to the disease, but in many populations the majority of the people will have been exposed to the virus in any event.  

Lassa fever at Wikipedia