An infectious disease is caused by what?

• A. A pathogenic organism that enters the body
• B. An environmental toxin
• C. A genetic mutation only
• D. A non-pathogenic bacteria

Answer: (A)

An infectious disease is caused by a pathogenic organism that enters the body and can multiply, evading the body's defenses to produce illness. Pathogens include bacteria, viruses, fungi, and parasites, all able to invade tissues and trigger symptoms. The emphasis is on the organism being capable of causing disease and entering the body, which distinguishes infectious diseases from other types of illness.

Environmental toxins cause disease through chemical injury, not infection. Genetic mutations can lead to inherited conditions, which are not infectious. Non-pathogenic bacteria don’t typically cause disease, so they don’t explain infectious conditions. For example, a virus like the flu enters the body and disrupts cellular function, leading to illness.