Humans are the only organism affected by the measles virus. What role in the chain of infection do humans play in this cycle?

Reservoir.

Explanation:

Measles is the infectious disease caused by the rubella virus. Measles can cause infectious or rashes in the body. The vaccination is available for the prevention of the measles.

The humans acts as the reservoir in the measles as humans provides the habitat in which the infectious agents grow. The reservoir may or may not transfer the infectious disease to the host. The humans that acts as reservoir may suffer from illness.

Thus, the answer is reservoir.

reservoir

Explanation:

Reservoir -

In the chain of infection,

A reservoir may include, animals and humans, i. e., in which the infectious agent lives, can multiply and grow, is referred to as a reservoir.

For many infectious disease, humans are the reservoir.

A reservoir may or may not be the main source of from which the infectious agent gets transferred to the host.

Hence, from the given information of the question,

The correct term is a reservoir.