Q. A 35-year-old male presents to the emergency department with a chief complaint of watery diarrhea and vomiting for the past 24 hours. The patient reports that he traveled to a developing country a week ago for business and consumed food and water from local vendors. He has no prior medical history of gastrointestinal illness or travel-related illnesses. On physical examination, the patient appears dehydrated with dry mucous membranes, a rapid heart rate, and low blood pressure. His abdomen is soft and nontender, with hyperactive bowel sounds. Laboratory tests show hyponatremia and hypokalemia. Stool cultures are sent to the laboratory for analysis. Based on the patient's history, clinical presentation, and laboratory findings, you suspect a diagnosis of cholera. The patient is immediately started on intravenous fluid therapy to correct his dehydration and electrolyte imbalances. Antibiotic treatment with doxycycline is initiated to target the underlying bacterial infection. The patient is also placed on droplet precautions and admitted to an isolation room to prevent the spread of infection. Which of the following factors does NOT increase susceptibility to this infection?