**Core Concept**
Weil's disease, also known as leptospirosis, is a zoonotic infection caused by bacteria of the genus *Leptospira*. The disease is characterized by a biphasic illness, with an initial febrile phase followed by a more severe, organ-specific phase.

**Why the Correct Answer is Right**
Leptospira bacteria are transmitted through contact with water or soil contaminated with the urine of infected animals, such as rats. The bacteria enter the body through cuts or abrasions in the skin or through mucous membranes. Once inside, *Leptospira* evade the immune system by inhibiting phagocytosis and inducing an inflammatory response. The bacteria then cause damage to various organs, including the kidneys, liver, and central nervous system.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as Weil's disease is not caused by a virus.
**Option B:** Leptospirosis is not caused by a fungus, making this option incorrect.
**Option C:** This option is incorrect as Weil's disease is not caused by a protozoan.

**Clinical Pearl / High-Yield Fact**
It's essential to note that Weil's disease can be prevented by avoiding contact with standing water or soil contaminated with animal urine, and by wearing protective clothing and gloves when handling animals.

**Correct Answer: D. Leptospira.**