Which of the following is a disease-causing organism?

• A. Virus
• B. Germ
• C. Pathogen
• D. Bacteria

Answer: (C)

Pathogen is the term for any organism that can cause disease in a host. This broad label covers viruses, bacteria, fungi, and parasites—any agent capable of making someone sick. In water-quality contexts, identifying a pathogen signals a disease-causing organism is present. A virus is a disease-causing organism too, but it’s only one type of pathogen, not the general term. Bacteria are another specific type, and some bacteria aren’t harmful, so naming just bacteria doesn’t capture all disease-causing agents. Germ is a colloquial, nonspecific word that doesn’t reliably define disease-causing potential. Therefore, the best fit for a disease-causing organism in general is pathogen.