- At least 72 people have died and 2,845 people at risk are being monitored
- "Many people will seek treatment in health facilities that are not appropriately prepared," official says
A further 764 are suspected to be infected, and 2,845 contacts have been identified.
On average, Lassa fever is deadly in 1% of all individuals infected, with higher rates of 15% morbidity among people hospitalized for the illness. As of Sunday, the case fatality ratio was 22%, according to the Nigeria Centre for Disease Control.
Although it's endemic to the country, Lassa fever numbers have never reached this proportion before, according to the WHO.
Nigeria's Centre for Disease Control
said Wednesday that it was facing an "unprecedented outbreak" that has spread to 18 states since it began in January.
The disease can cause fever and hemorrhaging of various parts of the body -- including the eyes and nose -- and can be spread through contact with an infected rat.
Person-to-person transmission is low but has been seen in Nigerian hospital settings this year. Fourteen health workers were infected, of whom four died within eight weeks.
The WHO said Wednesday
that health facilities were overstretched in the southern states of Edo, Ondo and Ebonyi, and it is working with national reference hospitals and the Alliance for International Medical Action to rapidly expand and better equip treatment centers.