What causes diseases?

• A. Pathogenic microorganisms
• B. Non-pathogenic microorganisms
• C. Viruses only
• D. Bacteria only

Answer: (A)

Pathogenic microorganisms are the agents that cause disease because they invade tissues, multiply, and disrupt normal body functions. They include bacteria, viruses, fungi, and parasites, each capable of damaging cells, triggering inflammation, or hijacking cellular processes. Non-pathogenic microorganisms do not cause disease and can even support health. Saying viruses only or bacteria only is too narrow, since diseases can be caused by fungi, parasites, and other pathogens as well.