Answer: a. True
Explanation: These postulates were formulated by Robert Koch as a result of his experiments with healthy and infected mice. The postulates are often used to determine if a phatogen causes a disease. These are:
- The pathogen must be present in sick individuals but not in healthy ones.
- The pathogen must be isolated from individuals and cultivated in a pure culture.
- The pathogen cultivated must cause sickness when it's injected in suceptible inviduals.
- The pathogen must be isolated from the injected individuals and must be exactly as the first one.