Cholera is caused by

Infectious diseases are classified by the biological nature of their causative agent: bacteria (single-celled prokaryotic organisms), viruses (sub-microscopic agents that replicate only inside a host's cells), insects or other vectors that carry a pathogen between hosts, and toxins (poisonous substances produced by a living organism as a downstream product of infection).

Cholera is an acute diarrhoeal infection caused by the bacterium Vibrio cholerae.

1. Vibrio cholerae is a gram-negative, comma-shaped bacterium that contaminates drinking water or food and spreads through the faecal-oral route.

2. After ingestion, the bacterium colonises the small intestine and multiplies there.

3. It secretes cholera toxin (CTX), which drives a massive efflux of water and electrolytes into the gut, producing profuse watery diarrhoea.

4. Left untreated, this rapid fluid loss causes severe dehydration, so prompt oral or intravenous rehydration is the core treatment.

Checking against the other categories confirms the bacterial classification:

• Viral gastroenteritis (for example, caused by rotavirus or norovirus) comes from a genetically distinct pathogen class that does not produce cholera toxin.

• Vector-borne insect transmission (for example, the Anopheles mosquito spreading the parasite that causes malaria) requires a bite to pass the pathogen, whereas cholera spreads through contaminated water or food, not an insect bite.

• Cholera toxin is a substance secreted by the bacterium after it has infected the gut; it is a product of the infection, not itself a self-replicating living organism, so "toxin" describes a mechanism of the bacterium rather than the causative-agent category.

Since the causative organism is a living, self-replicating bacterium transmitted through contaminated water or food, the correct classification is Bacteria.