Types of Pathogens

A pathogen is a microorganism that causes disease. Pathogens exploit the body's resources to reproduce, often damaging host tissues or producing toxins in the process.

Four Main Types of Pathogens

Bacteria

Single-celled prokaryotes that reproduce rapidly by binary fission. Some produce toxins that damage tissues. Examples: Streptococcus (strep throat), Mycobacterium tuberculosis (TB).

Treated with antibiotics

Viruses

Non-living particles with DNA or RNA inside a protein coat (capsid). They hijack host cell machinery to replicate. Examples: influenza, SARS-CoV-2, HIV.

Antibiotics are ineffective; treated with antivirals or prevented by vaccines

Fungi

Eukaryotic organisms that can cause superficial or systemic infections. They release enzymes that break down host tissue. Examples: Tinea (athlete's foot), Candida (thrush).

Treated with antifungals

Protists (Protozoa)

Single-celled eukaryotes, often with complex life cycles involving vectors. Examples: Plasmodium (malaria, spread by mosquitoes), Giardia (waterborne).

Treated with antiprotozoal drugs

Routes of Transmission

Direct contact: Touching infected person, bodily fluids (e.g. HIV, herpes).

Droplet/airborne: Inhaling respiratory droplets or aerosols (e.g. influenza, TB).

Faecal-oral: Contaminated food or water (e.g. cholera, hepatitis A).

Vector-borne: Carried by an organism such as a mosquito or tick (e.g. malaria, Lyme disease).

The Immune Response

The human immune system has multiple lines of defence. The innate immune system provides immediate, non-specific protection, while the adaptive immune system mounts a targeted, specific response that produces immunological memory.

Three Lines of Defence

First Line: Physical & Chemical Barriers

Skin, mucous membranes, tears (lysozyme), stomach acid, cilia in airways -- prevent pathogen entry

↓ If breached...

Second Line: Innate (Non-specific) Response

Inflammation, fever, phagocytes (neutrophils, macrophages) engulf and destroy pathogens, natural killer cells

↓ If infection persists...

Third Line: Adaptive (Specific) Response

Lymphocytes: B cells produce antibodies; T cells destroy infected cells. Creates memory cells for future immunity

Adaptive Immune Response

Humoral Immunity (B cells)

1. Antigens on the pathogen are recognised by specific B lymphocytes.
2. Activated B cells proliferate and differentiate into plasma cells.
3. Plasma cells secrete antibodies that bind to antigens, neutralising pathogens or marking them for destruction.
4. Memory B cells remain for rapid secondary response.

Cell-mediated Immunity (T cells)

1. Helper T cells (CD4+) recognise antigens presented by macrophages and activate B cells and killer T cells.
2. Killer T cells (CD8+/cytotoxic) directly destroy infected host cells.
3. Memory T cells provide long-term immunity for faster future responses.

Vaccination and Herd Immunity

Vaccination is the deliberate introduction of a weakened, inactivated, or fragment of a pathogen (or its mRNA instructions) to stimulate an adaptive immune response without causing disease. This produces memory cells that enable a rapid secondary response upon future exposure.

How Vaccination Works

Vaccine given

Contains antigen

→

Immune response

B and T cells activated

→

Memory cells

Long-lived memory formed

→

Future exposure

Rapid secondary response

Herd immunity: When a high proportion of a population is vaccinated (typically 80-95%, depending on the disease), the pathogen cannot spread easily, indirectly protecting those who cannot be vaccinated (e.g. immunocompromised individuals, infants).

Key Vocabulary

Antigen

A molecule (usually a protein) on the surface of a pathogen that is recognised as foreign by the immune system, triggering an immune response.

Antibody

A Y-shaped protein produced by plasma cells (B lymphocytes) that binds specifically to an antigen, neutralising the pathogen or marking it for destruction.

Phagocytosis

The process by which phagocytes (e.g. macrophages, neutrophils) engulf and digest pathogens and cellular debris.

Immunological Memory

The ability of the adaptive immune system to "remember" a pathogen via memory B and T cells, enabling a faster and stronger response upon re-exposure.

Worked Examples

1

Explain why antibiotics are effective against bacterial infections but not viral infections.

Step 1: Antibiotics target structures or metabolic processes found in bacteria (e.g. cell wall synthesis, protein synthesis on bacterial ribosomes, DNA replication).

Step 2: Viruses are not cells -- they lack cell walls, ribosomes, and their own metabolic machinery. They replicate using the host cell's machinery.

Step 3: Therefore, antibiotics have no bacterial target to act on within a virus. Antiviral drugs or vaccines are needed instead, as these target virus-specific processes (e.g. viral replication enzymes).

2

Describe the difference between the primary and secondary immune responses.

Primary response: First exposure to a pathogen. The adaptive immune system takes 7-14 days to activate, as naive B and T cells must recognise the antigen, proliferate, and differentiate. Antibody levels rise slowly.

Secondary response: Upon re-exposure to the same pathogen, memory cells are activated rapidly. The response is faster (1-2 days), stronger (higher antibody concentration), and longer-lasting.

This is why vaccination works: The vaccine triggers the primary response and memory cell formation, so if the real pathogen is encountered, the faster secondary response prevents disease.

3

Explain the concept of herd immunity and why it is important.

Step 1: When a large proportion of a population is immune (through vaccination or prior infection), the chain of transmission is broken because the pathogen encounters fewer susceptible hosts.

Step 2: This indirectly protects individuals who cannot be vaccinated (e.g. newborns, immunocompromised patients, those with allergies to vaccine components).

Step 3: The threshold for herd immunity depends on how contagious the disease is. Highly contagious diseases like measles require ~95% immunity, while less contagious diseases may require lower thresholds (~80-85%).

Knowledge Check

Select the correct answer for each question. Click "Check Answer" to see if you are right.

Question 1

Which type of pathogen requires a host cell's machinery to replicate?

Bacteria Fungi Protists Viruses

Question 2

Phagocytes such as macrophages are part of which line of defence?

Second line (innate, non-specific) First line (physical barriers) Third line (adaptive, specific) They are not part of the immune system

Question 3

Which cells produce antibodies during the adaptive immune response?

Helper T cells Plasma cells (differentiated B cells) Killer T cells Neutrophils

Question 4

Why is the secondary immune response faster and stronger than the primary response?

The pathogen is weaker the second time The skin becomes thicker after the first infection Memory B and T cells are already present and can be activated rapidly Phagocytes evolve to become more efficient

Question 5

Malaria is caused by Plasmodium and transmitted by mosquitoes. What type of pathogen is Plasmodium and what is the mosquito called?

Bacterium; the mosquito is the host Virus; the mosquito is the reservoir Fungus; the mosquito is the vector Protist (protozoan); the mosquito is the vector

Infectious Disease and Immunity