In Weil`s disease, the organ that gets affected is the kidney. Weil`s disease, also known as Leptospirosis, is a bacterial infection caused by the Leptospira bacteria. This infection is commonly transmitted through contaminated water or soil containing animal urine. When the bacteria enters the body, it can cause damage to various organs, including the kidneys.

The kidneys play a crucial role in filtering waste products from the blood and regulating fluid balance. In Weil`s disease, the bacteria can directly damage the kidney tissues, leading to kidney dysfunction. This can result in symptoms such as decreased urine output, dark-colored urine, fatigue, and swelling in the legs and ankles.

While the other organs mentioned in the options (liver, lung, and heart) can also be affected in severe cases of Weil`s disease, the primary organ affected is the kidney.