What characterizes a biological vector?

A biological vector is characterized by its role in the life cycle of a disease-causing organism, particularly parasites, bacteria, or viruses. The key feature of a biological vector is that the disease agent not only resides within the vector but also undergoes development or multiplication.

In this context, choice C accurately represents this characteristic as it indicates that the disease agent increases in number while in the vector. This multiplication or development often changes the vector's role from being merely a transport mechanism to that of an integral part of the disease's life cycle. For example, in the case of mosquitoes transmitting malaria, the malaria parasite undergoes stages of development within the mosquito before being transmitted to humans.

The other choices do not encapsulate the core definition of biological vectors. For instance, stating that a biological vector does not interact with the disease agent contradicts the very nature of biological vectors, which are defined by their interaction with and support of the disease agent. Claiming that the vector kills the disease agent overlooks the biological vector's role in facilitation rather than elimination. Lastly, the idea that a biological vector only transports the disease agent applies more accurately to mechanical vectors, which do not partake in the multiplication of the organism. Thus, the correct identification of a biological vector hinges