Final answer:
Communicable diseases are those that can be transmitted from one host to another, with contagious diseases being a subset that spreads easily among individuals.
Step-by-step explanation:
Agents and diseases that are capable of being transmitted to another host are referred to as communicable diseases. A contagious disease is a type of communicable disease that is very easily spread from person to person. Not all infectious diseases are communicable; some are harbored in environmental reservoirs or within animals, and humans become incidental hosts.
An example of a highly contagious disease is measles, which is spread through airborne transmission, whereas a disease like gonorrhea, which is caused by Neisseria gonorrhoeae, is less contagious and requires direct intimate contact to be transmitted. Infectious diseases are caused by various types of pathogens including bacteria, viruses, fungi, and protozoa.