An outbreak of Lassa fever in Nigeria has infected more than 350 people since January.
People who contract Lassa fever usually have mild or no symptoms, but in severe cases it can cause bleeding, deafness, or organ failure.
Lassa fever is normally spread to humans from rodents carrying the Lassa virus.
There is no vaccine for Lassa fever, and early treatment is critical.
Once a laboratory test confirms someone has Lassa fever, there are typically two types of treatment. One approach focuses on symptoms, Rollin says, which means getting the fever down and giving the person fluids and supportive care.
"There's also a treatment called Ribavirin [an antiviral drug] that seems to be effective when given early in the course of the disease," Rollin says. In about 25–30% of severe cases, patients will experience partial or complete hearing loss, for which there is no specific treatment or cure.