pathogen: the causative agent of a communicable disease. For example, influenza (disease) is caused by influenza virus (pathogen); malaria (disease) is caused by species of Plasmodium (pathogen). It is a common mistake to use the term for the disease when you actually mean the pathogen, and vice versa:
He contracted measles not He contracted measles virus
The sample was tested for measles virus not The sample was tested for measles
The sample was tested for measles virus not The sample was tested for measles
pathology: the study of diseases; do not use to mean disease
Influenza is a disease not Influenza is a pathology