The text book I'm teaching from states for infectious respiratory diseases you code the diseases of the respiratory system (460-519) them use additional code to identify infectious organism, ex pneumonia adenoviral 480.0 and infection adenovirus 079.0. My question is when coding COPD with asthma you code 493.0 no additional code is needed. Same with COPD with bronchitis you use 1 code 491.22. Is this due to the word infectious? How would you determine which diseases are infectious? Could you use rational if the disease is contagious it's infectious?