An interesting discussion from This American Life (“The Herd“, transcript) talking about the origin of the term “communicable disease” with respect to public health monitoring for the 1918 flu pandemic.
Ira Glass: Howard says [St. Louis Commissioner of Health] Dr. Starkloff had all kinds of messages, telling people to wash their hands, don’t cough on others. And because medicine was so primitive back then, they hadn’t actually isolated the flu virus. So they had no way to confirm if you had it. They couldn’t test you. So the only way they could keep track of the disease was that people who were sneezing and sick told authorities. So Starkloff would tell the public–
Howard Markel: These are the symptoms that you need to call us about. You need to communicate this to us. That’s where the word “communicable disease” comes from, by the way. Not so much that it’s infectious, even though it is, but that one source is communicating it to a higher source and to a public health department.
Ira Glass: Oh, wow.
Howard Markel: Yeah. So that works because if you can get people on board with that, they’re self-isolating, they’re self-reporting, they’re self-quarantining and so on. So that’s good.
Thus, while communicable disease now is largely synonymous with infectious disease, the origin comes from the need to communicate symptoms to public health officials for disease monitoring purposes.