One estimate places  the number as high as 3 million cases per year.Estimates of Lassa fever are complicated by the lack of easy-available diagnosis, limited public health surveillance infrastructure, and high clustering of incidence near high intensity sampling.The infection affects females 1.2 times more than males. The age group predominantly infected is 21–30 years. Geography
Lassa high risk areas are near the western and eastern extremes of West Africa. As of 2018, the Lassa belt includes Guinea, Nigeria, Sierra Leone and Liberia. As of 2003, 10-16% of people in Sierra Leone and Liberia admitted to hospital had the virus. The case fatality rate for those who are hospitalized for the disease is about 15-20%. Research showed a twofold increase risk of infection for those living in close proximity to someone with infection symptoms within the last year. The high risk areas cannot be well defined by any known biogeographical or environmental breaks except for the multimammate rat, particularly Guinea (Kindia, Faranah and Nzérékoré regions), Liberia (mostly in Lofa, Bong, and Nimba counties), Nigeria (in about 10 of 36 states) and Sierra Leone (typically from Kenema and Kailahun districts). It is less common in the Central African Republic, Mali, Senegal and other nearby countries, and less common yet in Ghana and the Democratic Republic of the Congo. Benin had its first confirmed cases in 2014, and Togo had its first confirmed cases in 2016.As of 2013, the spread of Lassa outside of West Africa had been very limited.
It is possible to acquire the infection through broken skin or mucous membranes that are directly exposed to infectious material. Transmission from person to person has been established, presenting a disease risk for healthcare workers. The virus is present in urine for between three and nine weeks after infection, and it can be transmitted in semen for up to three months after becoming infected. Diagnosis
A range of laboratory investigations are performed, where possible, to diagnose the disease and assess its course and complications. The confidence of a diagnosis can be compromised if laboratory tests are not available. One comprising factor is the number of febrile illnesses present in Africa, such as malaria or typhoid fever that could potentially exhibit similar symptoms, particularly for non-specific manifestations of Lassa fever. In cases with abdominal pain, in countries where Lassa is common, Lassa fever is often misdiagnosed as appendicitis and intussusception which delays treatment with the antiviral ribavirin. In West Africa, where Lassa is most common, it is difficult to diagnose due to the absence of proper equipment to perform testing.The FDA has yet to approve a widely validated laboratory test for Lassa, but there are tests that have been able to provide definitive proof of the presence of the LASV virus. These tests include cell cultures, PCR, ELISA antigen assays, plaque neutralization assays, and immunofluorescence essays. However, immunofluorescence essays provide less definitive proof of Lassa infection. An ELISA test for antigen and Immunoglobulin M antibodies give 88% sensitivity and 90% specificity for the presence of the infection.