Pathogens as the cause of infectious diseases

Organisms (Interaction and interdependence) · Defence against disease · note 1 of 8

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Pathogens as the cause of infectious diseasesSpec C3.2.1

In short

A pathogen is a disease-causing organism, and pathogens are the cause of infectious diseases. The term is typically reserved for viruses, bacteria, fungi and protists, although a broad range of organisms can infect humans. Archaea are not known to cause any diseases in humans. Careful observation of epidemics led to major progress in controlling infectious disease.

A pathogen is a disease-causing organism. Pathogens are the cause of infectious diseases: diseases that can be passed from one host to another. A broad range of organisms can infect humans, but the term pathogen is typically reserved for four groups.

Groups of pathogen
Group	Example disease
Viruses	COVID-19, HIV/AIDS, rabies
Bacteria	Tuberculosis, cholera
Fungi	Athlete's foot
Protists	Malaria

Archaea are not known to cause any diseases in humans, even though many live in and on the human body.

Nature of science: careful observation

Careful observation can lead to important progress. In 19th-century Vienna, a doctor observed that childbed fever (an infection after childbirth) was far more common on a ward where doctors came straight from dissecting bodies; making them wash their hands in a chlorinated solution cut deaths sharply. In London, mapping cholera cases during an epidemic showed that they clustered around one water pump, linking the disease to contaminated water. Both led to breakthroughs in the control of infectious disease.

Written and checked against the IB Biology HL specification · Updated October 2026

Frequently asked questions

How do vaccines work?

Vaccines contain antigens, or DNA or RNA coding for antigens, from a specific pathogen. They trigger a primary immune response in which B-cells are activated and memory cells are produced, without causing the disease. If the real pathogen infects later, memory cells respond quickly, producing antibodies before the person becomes ill.

Why can't antibiotics kill viruses?

Antibiotics work by blocking processes found in bacteria but not in eukaryotic cells, such as cell wall synthesis or protein synthesis by 70S ribosomes. Viruses have no metabolism of their own, so there is nothing for antibiotics to block. They replicate using the host cell's processes, which antibiotics do not target.

What is the difference between innate and adaptive immunity?

The innate immune system responds to broad categories of pathogen and does not change during an organism's life; phagocytes are part of it. The adaptive immune system, based on lymphocytes, responds specifically to particular pathogens and builds up a memory of pathogens encountered, so later responses are faster and more effective.

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