What is an example of an organism that acts as a biological vector?

A biological vector is an organism that transmits pathogens from one host to another, facilitating the spread of diseases. In this case, a flea serves as a biological vector because it can carry and transmit various pathogens, including bacteria and parasites, that can cause diseases in other organisms, such as mammals.

Fleas, particularly in their role as vectors, can transmit conditions like the plague or murine typhus to their hosts by biting. In this capacity, they not only serve as a carrier but are often essential to the life cycle of the pathogens they transmit, making them effective vectors.

Other organisms listed may interact with diseases or pathogens in different ways. For example, while rodents can be hosts for various pathogens, they do not act primarily as vectors transferring diseases in the same way fleas do. Similarly, humans, while carriers of many diseases, typically do not fulfill the role of a biological vector in the same context, and earthworms do not transmit diseases. This differentiates them from fleas, which actively facilitate the spread of infectious agents.