Pathological changes in the lungs were graded based on the following criteria: pathological characterization of the severity of lung damage in SARS Rhesus macaques

Grade	Pathological characterization of severity of lung damage in SARS Rhesus macaques
−: Normal lung section of macaque without SARS-CoV infection
±: Minor inflammation, slight broadening of alveolar septa, and sparse monocyte infiltration
+: Apparent inflammation, hemorrhage in septa, elastic fibers of alveolar wall distorted, as shown by silver staining
++: Early symptoms of acute DAD, alveolar septal broadening with increasing inflammatory cell infiltration
+++: Typical symptoms of acute DAD, extensive exudation and septal broadening, shrinkage of alveoli caused by pressure, restricted fusion of thick septa, ruptured elastic fibers of alveoli, variably filled with protein-rich edema fluid, fibrin, erythrocytes, cellular debris, and a moderate number of inflammatory cells in alveolar cavities
++++: Severe acute DAD, massive cell infiltration and alveolar shrinkage, sheets of septal fusion, necrotic lesions at the hemorrhagic septa, and massive cell infiltration in alveolar cavities

NOTE. DAD: diffuse alveolar damage.