Almost all viruses enter the body via the oropharynx as droplet or by licking contaminated surfaces. They then replicate in the URT. Some which replicate only in respiratory epithelial cells can only spread down the tract e.g. Respiratory Syncytial virus.

The invasiveness of the virus determines how far it goes down eg FCV remains in the URT and FVR extends to the bronchioles.

Other viruses which can replicate in the reticulendothelial system use the URT as an entry port eg canine distemper.

EHV 1 and 4) cause a transient rhinopneumonitis with cough necessitating rest and antibiotic cover. The more virulent abortigenic strains of equine herpes 1 can also infect vascular endothelial cells and lymphocytes. They thrombose blood vessels to cause either a placentitis in the 3rd trimester)- the feared abortion storm- or hemorrhages in the spinal cord (posterior paresis).

Avirulent NDV and Fowl Plaque isolates replicate in the URT.