Final answer:
The correct definition of a pathogen in the provided options is that fungi are pathogens requiring a living host to grow. Pathogens can cause disease by invading and multiplying within host tissues, but not all grow rapidly in food with high moisture content, and not all are invisible without a microscope. So the correct option is C.
Step-by-step explanation:
The statement defining a true pathogen is: C. fungi are pathogens that need a living host to thrive and grow. Pathogens include bacteria, fungi, and protozoa, among others, and they cause disease by invading and multiplying in host tissues. It's also essential to understand that not all pathogens will grow rapidly in foods with high moisture content, and while many are invisible to the bare eye and require a microscope to be seen, this is not a definition of a pathogen but rather a characteristic of many microscopic organisms.
Pathogens cause disease by invading and multiplying in host tissues, damaging the host, and sometimes releasing toxins. Their virulence can vary, which affects the severity of the diseases they cause. Pathogen transmission can occur in many ways, including direct contact, ingestion, and airborne transmission.