**Core Concept**
Traveller's diarrhoea is a common condition affecting individuals who travel to areas with different sanitation and hygiene standards. It is primarily caused by the ingestion of contaminated food and water, leading to an imbalance in the gut microbiota. The condition is often characterized by a sudden onset of watery stools, abdominal cramps, and fever.

**Why the Correct Answer is Right**
The most common cause of traveller's diarrhoea is bacterial infection, particularly by enterotoxigenic *Escherichia coli* (ETEC). ETEC produces heat-labile (LT) and heat-stable (ST) toxins that stimulate the secretion of water and electrolytes into the intestinal lumen, resulting in diarrhoea. This mechanism is mediated through the activation of adenylate cyclase, leading to an increase in cyclic AMP levels within the intestinal epithelial cells.

**Why Each Wrong Option is Incorrect**
* **Option A:** Rotavirus is a common cause of diarrhoea in children, but it is not the primary cause of traveller's diarrhoea.
* **Option B:** *Salmonella* species can cause traveller's diarrhoea, but they are less common than ETEC.
* **Option C:** *Shigella* species can also cause traveller's diarrhoea, but they are less common than ETEC.

**Clinical Pearl / High-Yield Fact**
The use of antibiotics such as ciprofloxacin can be effective in treating traveller's diarrhoea caused by bacterial pathogens, but their use should be judicious and guided by stool culture results.

**Correct Answer:** C. *Salmonella* species