Five-Day Disease

Five-Day Disease is a zoonotic disease caused by a bite from the tick Kestia. Nami caught this disease during the crew's stay in Little Garden, and was later cured by Kureha and Chopper in the Drum Castle.

Kestia
The tick with the name Kestia is responsible for the Five-Day Disease. It inhabits tropical islands and existed in pre-historic times. Therefore, it is a rare animal thought to be extinct that exists in Little Garden due to the island's ancient fauna. Its bite leaves a huge mark, as seen on the left side of Nami's abdomen, and causes the victim's death in five days.

Symptoms
The first symptoms shown by Nami were exhaustion and sweating. Shortly after, she fainted with blushing cheeks and forehead as well as a high fever, which rose to 40 °C, something Vivi points out almost never happens. After getting up from bed, Nami reveals to Zoro she was having a headache.

Three days since the beginning of the disease, in Drum Island, Nami had a fever of 42 °C that was revealed to be steadily getting higher.

In the care of Kureha and Chopper, the mark left by Kestia's bite was shown as multiple circles surrounding a bigger one. Kureha explained that within five days, the disease causes failure of the heart, brain and eventually all parts of the victim's body. Therefore, five days is how long a victim can stay alive.

Cure
As an attempt by the Straw Hats to ease Nami's condition, she was bedridden during most of their travel to Drum Island, with a towel over her head and a blanket over her body. Sanji made sure to cook nutritious food that would be easy to digest. However, these measures did not show any major contributions to Nami's health.

After Luffy was able to carry Nami to the Drum Castle, Nami was treated by Kureha and Chopper. Nami's temperature lowered to 38.2 °C due to the medicine Kureha still had for the condition. After Nami questions if she would leave soon, Kureha explains that normally it would take ten days to be completely cured, but her medicine is able to lower that time to three days. Even if Nami had to be carried by Luffy to the castle, she was able to run and walk normally by the end of the Straw Hats' stay in Drum Island.

Trivia

 * The disease appears not to be transmissible from human to human, as multiple characters had contacted with Nami when she was sick, yet none showed symptoms; and Kureha, who knew about the disease, didn't take measures to stop transmission.