​Select an option which is different from the rest. ​

​​Let's analyze the given options:

Fever – A symptom rather than a specific disease.

Influenza – A viral disease caused by the influenza virus.

Yellow fever – A specific viral disease caused by the yellow fever virus.

Smallpox – A specific viral disease caused by the variola virus.

So, Influenza, Yellow fever, and Smallpox are all specific viral diseases.

Fever, however, is just a symptom and not a disease by itself.

Thus, correct option is (a).