1.4: Pathogens, including viruses

LEARNING GOALS

By the end of this lesson

Define pathogen and identify the four pathogen groups required by the course.
Describe the shared structural features of viruses.
Explain why viruses replicate only inside living cells.

BIG QUESTION

What does a virus borrow from a host cell that it cannot provide for itself?

Pathogen describes the capacity to cause disease; virus describes an acellular particle with genetic material in a protein coat.

Pathogen is a role, not a kingdom

A is a microorganism or infectious agent (including viruses) that causes disease. Pathogens in this course may be fungi, bacteria, protoctists or viruses. The word describes their effect on a host, not one shared cellular structure.

Most members of a group are not necessarily pathogenic. Many bacteria are harmless or beneficial; many fungi are decomposers; and many protoctists do not infect humans. Use “pathogenic” only when evidence shows that a particular organism or agent causes disease.

Viruses are acellular particles

A virus is not a cell. Every virus contains genetic material—either DNA or RNA—inside a called a capsid. Some viruses also have a lipid envelope derived from a host membrane, but an envelope is not present in every virus.

Viruses have no cytoplasm, ribosomes or independent metabolism. They are much smaller than typical bacteria. Surface proteins bind to particular molecules on host cells, helping determine which cells they can infect.

Replication requires a host

A virus cannot make proteins or copy itself independently. After entering a suitable host cell, its genetic material redirects host enzymes, ribosomes, nucleotides and amino acids to produce viral components. These components assemble into new virus particles.

Because the machinery belongs to the host, viral replication is not cell division. New particles may leave by bursting the cell or by budding through its membrane, depending on the virus.

Course examples

Influenza viruses infect cells lining the respiratory tract. Human immunodeficiency virus, , targets immune cells; untreated infection can progress to AIDS. Tobacco mosaic virus, , infects plants and produces a mosaic pattern that reduces photosynthetic leaf area.

Plasmodium is a protoctist pathogen transmitted by female Anopheles mosquitoes. Trichophyton is a fungal pathogen associated with athlete’s foot. Pneumococcus is a bacterial pathogen that can cause pneumonia. These examples separate the groups clearly.

Why viruses sit outside the living groups

Viruses possess genes and populations evolve, but a virus particle does not independently respire, grow, excrete or reproduce. It depends on the metabolism of a living host cell.

For this specification, viruses are treated as particles rather than members of the living groups. A strong answer acknowledges the life-like evidence, then explains the decisive lack of cellular organisation and independent metabolism.

Four sources of infectious disease

Classify the pathogen before describing how it causes or spreads disease.

Pathogen groups and representative course examples
Pathogen group	Cellular status	Representative example
Fungus	Eukaryotic cell or hyphae with chitin walls.	Trichophyton causing athlete’s foot.
Bacterium	Living prokaryotic cell.	Pneumococcus causing pneumonia.
Protoctist	Living eukaryotic cell.	Plasmodium causing malaria.
Virus	Acellular particle containing DNA or RNA in a protein coat.	Influenza virus, HIV or TMV.

KEY TERMS

Language that earns precision

pathogen: A microorganism or infectious agent (including viruses) that causes disease.
virus: An acellular infectious particle containing DNA or RNA inside a protein coat.
capsid: The protein coat surrounding viral genetic material.
host: The organism or cell used by a parasite or pathogen.
replication: The production of new copies; viral replication requires a living host cell.
vector: An organism that carries a pathogen between hosts, such as a mosquito carrying Plasmodium.

Common misconceptions

Claim: All microorganisms are pathogens.

Correction: A microorganism is microscopic; a pathogen specifically causes disease.

Claim: Viruses are tiny bacteria.

Correction: Bacteria are prokaryotic cells. Viruses are acellular particles without cytoplasm or ribosomes.

Claim: A virus divides into two.

Correction: Viral components are made by host machinery and assembled into new particles.

Claim: HIV and AIDS are the same thing.

Correction: HIV is the virus; AIDS is a late-stage syndrome that untreated HIV infection can cause.

EXAM LANGUAGE

Match the answer to the command word

Define

Give the exact biological meaning. For pathogen, include that it causes disease.

Describe

For a virus, identify genetic material and protein coat before optional features.

Explain

Link the lack of ribosomes or metabolism to dependence on host-cell machinery.

CHECK YOUR UNDERSTANDING

Recall, apply, then justify

Viral genetic material is copied and new particles are produced in a host.
The virus contributes genetic instructions and can evolve.
The host supplies ribosomes, enzymes, energy and raw materials.
The virus does not become cellular or independently metabolically active, so the claim remains misleading for this course.

END OF THE LESSON

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