In these cases the virus rapidly destroys the lungs' alveoli, where gas transfer occurs, often causing acute respiratory distress syndrome (ARDS), which usually kills in half of all cases.

Hendra should be suspected if there is also the proximity of bats, or the presence of human cases of acute respiratory distress syndrome or encephalitis.

Etiologies of respiratory distress are numerous and include pneumonia, acute respiratory distress syndrome, pulmonary embolism, and exacerbations of chronic obstructive lung disease.