Let's break down the sentence:
* Parvo: The subject of the sentence (a type of virus)
* is: The verb
* a common disease: Describes the subject, with "common" being the adjective.
* in dogs: Indicates where the disease is found.
* caused by a virus: Explains the origin of the disease.
* in the air: Describes where the virus is found.
The sentence could be rephrased as: Parvo is a disease that is commonly found in dogs.