*Caused by a [[Circoviridae|porcine circovirus]] alone or in combination with [[Parvoviridae|porcine parvovirus]]
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*May cause mild [[Lungs Inflammatory - Pathology#Interstitial pneumonia|interstitial pneumonia]], failure of lungs to collpse on opening the thoracic cavity
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*Microscopically: thickening of alveolar wall due to type 2 pneumocyte hyperplasia