**Core Concept**
Relapsing fever is a disease caused by various species of Borrelia bacteria. It is transmitted to humans through the bite of an infected soft tick (genus Ornithodoros).

**Why the Correct Answer is Right**
The soft tick (Ornithodoros species) is the primary vector for transmitting relapsing fever. The bacteria, Borrelia, are ingested by the tick during its feeding on an infected animal or human. When the infected tick bites another human, it regurgitates the bacteria into the bloodstream, causing the disease. The distinctive feature of relapsing fever is its periodic relapses, which occur due to the tick's ability to transmit the bacteria in a cyclic pattern.

**Why Each Wrong Option is Incorrect**
**Option B:** KFD (Kyasanur Forest Disease) is transmitted by the bite of an infected hard tick (Hyalomma marginatum), not a soft tick.

**Option C:** Tularemia is primarily transmitted through the bite of an infected tick (Dermacentor andersoni), contact with infected animals, or contaminated water. It is not specifically associated with soft ticks.

**Option D:** Indian tick typhus is transmitted by the bite of an infected hard tick (Haemaphysalis spinigera), not a soft tick.

**Clinical Pearl / High-Yield Fact**
Soft ticks are nocturnal, burrowing ticks that typically feed on small mammals and birds, and are often found in caves, rock crevices, and other hidden areas.

**Correct Answer:**
✓ Correct Answer: A. Relapsing fever