Invasion of the host RESPIRATORY SYSTEM by microorganisms, usually leading to pathological processes or diseases.

Synonyms: Respiratory Infection, Upper, Respiratory Infections, Infections, Respiratory, Infection, Respiratory Tract, Infections, Upper Respiratory, Upper Respiratory Tract Infections, Upper Respiratory Tract Infection, Respiratory Tract Infections, Infections, Upper Respiratory Tract, Infections, Respiratory Tract, Upper Respiratory Infections, Respiratory Tract Infection