Infectious agents: These are the various organisms that are responsible for causing diseases. They are pathogenic in nature and form the first causes of disease.
They need to be categorized because:
Many disease-causing microorganisms have some common traits or have similar biochemical pathways.
These similar traits help in identifying the drug or antibiotic which helps in treating the diseases.
Additionally, it serves as a basis for identifying and classifying any novel infectious agents by tallying their characteristics to these known traits.