Context: Infectious diseases are sometimes called contagious disease when they are easily transmitted by contact with an ill person or their secretions (e.g., influenza). Thus, a contagious disease is a subset of infectious disease that is especially infective or easily transmitted. Other types of infectious/transmissible/communicable diseases with more specialized routes of infection, such as vector transmission or sexual transmission, are usually not regarded as "contagious", and often do not require medical isolation (sometimes loosely called quarantine) of victims. However, this specialized connotation of the word "contagious" and "contagious disease" (easy transmissibility) is not always respected in popular use.

Question: When are infectious diseases called contagious diseases? 
Answer: when they are easily transmitted by contact with an ill person

Question: What is a contagious disease a subset of?
Answer: infectious disease

Question: What sets a contagious disease after from a standard infectious disease?
Answer: especially infective or easily transmitted

Question: Diseases with vector transmission or sexual transmission don't often require what type of isolation?
Answer: medical

Question: What is not always respected in popular use?
Answer: specialized connotation of the word "contagious"

Question: When are infectious diseases called impossible diseases?
Answer: Unanswerable

Question: What is a contagious disease more important than?
Answer: Unanswerable

Question: What sets a contagious disease equal to a standard infectious disease?
Answer: Unanswerable

Question: What is always respected in popular use?
Answer: Unanswerable