As a field pathology is the study of the effects of pathogens, or disease causing organisms. It includes or is closely related to:
As you can see it is a vey large field, and there is a large overlap with other fields of biology and medicine, so most pathology research is refered to by its subdiscipline.
Pathologists are often found in hospitals as they are important for the diagnosis of disease, as they can examine tissue samples and are proficient at identifying a disease from how it affects different tissues. The way a disease affects the host is called the pathology of the disease.
Like many of the biological sciences pathology has expanded greatly over the the last 70 years, as invention of more and more advanced technologies such as the electron microscope and radioactive tracing enabled pathologists to see the actions of pathogens in much greater detail. In the last 10 years, the increasing number of antibiotic resistant micro-organisms has lead to an even greater need for pathology research to discover new antibiotics.