The o'nyong nyong virus (ONNV), which stems from the Semliki Forest virus serocomplex, is an alphavirus closely related to the chikungunya virus (CHIKV), the Ross River virus (RRV), and the Mayaro virus (MAYV). ONNV is endemic in sub-Saharan Africa, transmitted by the bite of the Anopheles funestus and Anopheles gambiae mosquitos, and will typically cause sporadic and massive large-scale epidemics. Several cases have been reported in sub-Saharan Africa in the last 70 years, including outbreaks in Uganda and the Ivory Coast. It is likely that the virus has spread to a greater territory, but limited access to diagnostic tools as well as misdiagnosis as CHIKV may have contributed to a downplay of the estimation of the ONNV's spread. More than 2 million ONNV cases have been recorded; the incidence of infection currently seems to follow a downward trend.

Anopheline mosquito vectors (A funestus and A gambiae) are usually found around large bodies of water. The possibility of Aedes spp being additional vectors for ONNV has also been suggested. Transmission can follow endemic and enzootic cycles. Furthermore, familial clustering seems to play a great role in transmission.

ONNV infection is characterized by a nonhemorrhagic dengue-like clinical syndrome. Most patients who seek medical care present with fever, myalgia, noneffusive polyarthropathy with polyarthritis-polyarthralgia primarily in the large joints, headaches, posterior cervical lymphadenopathy-lymphadenitis, and a generalized, usually pruritic, maculopapular rash. Hemorrhagic manifestations are rare and include epistaxis and gingivorrhagia. Conjunctivitis is also a rare feature of ONNV infection. The case definitions associated with the highest specificity and sensitivity with laboratory-confirmed results combine symptoms of fever, lymphadenopathy, and polyarthralgia.

The incubation period is estimated to last 8 days, following which patients are incapacitated for a median time of 4 days. There are no reports of fatality due to ONNV; however, cases of spontaneous abortion have been associated with infection. ONNV disease is self-limited, but infection is responsible for morbidity and incapacity to work for several days, principally due to polyarthropathy. In some cases, polyarthropathy persists, with long-lasting, debilitating polyarthralgia.

Some studies estimate the ratio of apparent infection to asymptomatic infection is 2:1, but others argue that this is likely an underestimation.

No sex or age group seems to be more affected than another.