Medical microbiology — Which group is directly responsible for causing infectious diseases in hosts?

Introduction / Context:
Infection occurs when a disease-causing agent invades and multiplies in a host. Distinguishing the causative agents from the host's defensive components is fundamental in microbiology and epidemiology.

Given Data / Assumptions:

• Agents may be viruses, bacteria, fungi, protozoa, or helminths.
• Host immune cells are responders, not causes, of disease.
• Terminology must be precise for clinical communication.

Concept / Approach:
Pathogens are organisms or particles capable of causing disease. Immune cells like T cells, lymphocytes (a broader category), and macrophages protect the host by recognizing and eliminating pathogens; they are not themselves causative agents of infectious disease.

Step-by-Step Solution:

Verification / Alternative check:
Case definitions in public health center on identifying the pathogen (e.g., culture, PCR, antigen tests) as the cause of disease.

Why Other Options Are Wrong:

• T cells, lymphocytes, macrophages are host defenses.
• Cytokines are signaling molecules; dysregulation can contribute to pathology but they are not infectious agents.

Common Pitfalls:
Confusing autoimmunity or immunopathology with infection; while immune responses can cause symptoms, the initiating cause in infectious disease is the pathogen.

Final Answer:
Pathogens