Kestia

The Kestia (translated as keschia in the Viz manga) is a type of tick that inhabits hot, humid jungles and carries bacteria in its bite. Once bitten an infection forms and after 5 days the infection begins to spread, causing the victim to experience intense pain. During this time, the victim’s temperature rises up to 40°c causing Myocarditis, Ateritis and Encephalitis. After the full 5 days of infection, the patient apparently dies. It takes 10 days after treatment to recover from the diease. While the Straw Hat Pirates were on Little Garden, Nami is bitten by a Kestia causing her to become sick. Kestia was supposed to have become extinct 100 years ago, but it survived on Little Garden because of its prehistoric setting.

Trivia
In the 4Kids dub, the whole of Little Garden was removed, but oddly kept the part about the Kestia or "Stingy-Dingy Bug".

Although Nami was bitten by the Kestia off-panel in the manga, she was bitten in the anime in Episode 77.