Feline viral rhinotracheitis (FVR), also called feline herpesvirus type 1 (FHV-1)

Accounts for the majority of acute upper respiratory infections in cats; tends to affect eyes and nose; see text for signs

Feline calicivirus (FCV)

Often hard to distinguish from feline viral rhinotracheitis; tends to affect mouth and lungs; see text for signs

Chlamydia infections (Chlamydia felis and Chlamydia psittaci)

Conjunctivitis, sneezing, eye discharge, fever may develop; rarely causes pneumonia (feline pneumonitis); of lesser importance than FHV-1 and FCV

Mycoplasma infections

Conjunctivitis and inflammation of the nasal lining (rhinitis); of lesser importance than FHV-1 and FCV

Feline infectious peritonitis virus

May cause signs of mild upper respiratory tract infection; usually causes body-wide illness

Reoviruses

Virus replicates in respiratory tract; may not always cause respiratory signs