Final answer:
Vibrio cholerae produces cholera toxin and causes cholera when it is lysogenic, as it acquires the toxin gene from the CTX phage through specialized transduction, leading to the severe diarrhea characteristic of the disease.
Step-by-step explanation:
Vibrio cholerae becomes capable of causing cholera when it is in the lysogenic phase because this is when the bacterium acquires the gene for cholera toxin (CT) through a process called specialized transduction. The CTX phage, a lysogenic filamentous bacteriophage, carries the cholera toxin gene and integrates it into the V. cholerae chromosome as a prophage. Cholera toxin, composed of an A subunit and five B subunits, disrupts normal cellular function by binding to intestinal epithelial cells and causing a cascade of biochemical events leading to the secretion of excessive amounts of fluid and electrolytes into the intestine, producing the "rice-water stool" diarrhea characteristic of cholera.