Rudolf Virchow
Rudolf Virchow was a German physician, anthropologist, and pathologist who made significant contributions to the field of medicine. However, he did not establish that a pathogen is a disease agent.

Robert Brown
Robert Brown was a Scottish botanist who discovered and described the nucleus of plant cells. While his work was important in the field of cell biology, he did not establish that a pathogen is a disease agent.

Edward Jenner
Edward Jenner was an English physician and scientist who is known for developing the smallpox vaccine. His work in the late 18th century laid the foundation for the concept of immunization and the use of vaccines to prevent infectious diseases. However, he did not specifically establish that a pathogen is a disease agent.

Robert Koch
Robert Koch was a German physician and microbiologist who is considered one of the founders of modern bacteriology. He is best known for his work on the identification and characterization of disease-causing microorganisms, particularly the bacteria that cause tuberculosis and cholera. Koch developed a set of criteria, known as Koch's postulates, which are used to establish a causal relationship between a specific microorganism and a particular disease. These postulates provide a framework for determining whether a specific pathogen is the cause of a given disease.

Koch's postulates state that:
1. The microorganism must be found in abundance in all organisms suffering from the disease but should not be found in healthy organisms.
2. The microorganism must be isolated from a diseased organism and grown in pure culture.
3. The cultured microorganism should cause disease when introduced into a healthy organism.
4. The microorganism must be re-isolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.

By applying these postulates, Koch was able to establish that certain bacteria were indeed the causative agents of specific diseases. His work revolutionized the understanding of infectious diseases and paved the way for the development of effective treatments and preventive measures.

In conclusion, the biologist who established that a pathogen is a disease agent is Robert Koch. Through his work on bacteria and the development of Koch's postulates, he provided a scientific framework for identifying and understanding the role of pathogens in causing diseases.