If you are living with chronic hepatitis B and dealing with unexplained joint pain, swelling, or morning stiffness, you are not imagining a connection. Hepatitis B virus does not always confine its damage to the liver.

O'nyong-nyong fever is one of those infections that sounds obscure until it affects someone you know — or until your own joints refuse to cooperate weeks after the fever has gone. Named from the Acholi language for "weakening of joints," this alphavirus illness swept through East Africa between 1959 and 1962, affecting an estimated two million people in what remains one of the largest mosquito-borne epidemics in recorded history.

Mayaro fever rarely makes headlines outside of tropical medicine circles, yet for people living in or returning from South American rainforest regions, it can mean weeks of debilitating joint pain, exhaustion, and fever that lingers far longer than expected.