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

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