Which of the following is most likely to cause disease in humans?

• A. Non-virulent viruses
• B. Commensal bacteria
• C. Pathogenic bacteria
• D. Saprophytic fungi

Answer: (C)

Verified step by step guidance

1. Step 1: Understand the definitions of each term to identify their relationship with human disease. Non-virulent viruses are viruses that do not cause disease, commensal bacteria live harmlessly on or in the human body, pathogenic bacteria are bacteria that cause disease, and saprophytic fungi feed on dead organic matter and typically do not cause disease in humans.
2. Step 2: Analyze the role of non-virulent viruses. Since they are non-virulent, they lack the ability to cause disease, so they are unlikely to be the cause of human illness.
3. Step 3: Consider commensal bacteria. These bacteria coexist with humans without causing harm, often providing benefits, so they are generally not disease-causing agents.
4. Step 4: Evaluate pathogenic bacteria. By definition, these bacteria have mechanisms to invade the host, evade the immune system, and cause disease symptoms, making them the most likely cause of disease among the options.
5. Step 5: Review saprophytic fungi. These fungi decompose dead matter and rarely infect healthy humans, so they are less likely to cause disease compared to pathogenic bacteria.