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What are pathogens, and how do the structures and life cycles of bacteria and viruses cause infectious disease?

Describe the structure of bacteria and viruses as pathogens and explain how they cause infectious disease

A focused answer to the H2 Biology Infectious Disease and Immunity outcome on pathogens. The structure of bacteria and viruses, how each causes disease (toxins and tissue damage versus host-cell hijacking), routes of transmission, and the meaning of a pathogen.

Generated by Claude Opus 4.88 min answer

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
  4. Try this

What this dot point is asking

SEAB wants you to describe the structure of bacteria and viruses as pathogens and to explain how each causes infectious disease, including transmission. You should be able to contrast the living bacterial cell with the non-cellular virus and link their structures to how they harm the host. This sets up the immune response that follows.

The answer

What is a pathogen?

A pathogen is a microorganism (or virus) that causes disease in its host. The main groups are bacteria, viruses, fungi and protoctists; the syllabus focuses on bacteria and viruses.

Bacteria

A bacterium is a living prokaryotic cell: a cell surface membrane, a peptidoglycan cell wall, circular DNA in a nucleoid (often with plasmids), and 70S ribosomes. Bacteria can reproduce on their own by binary fission.

Bacteria cause harm by multiplying in the host and often by releasing toxins that damage host cells or disrupt their function, and by directly damaging tissues.

Viruses

A virus is not a cell. It is a particle of genetic material (DNA or RNA) enclosed in a protein coat (the capsid), sometimes with a lipid envelope. A virus cannot reproduce by itself.

A virus causes harm by entering a host cell, using the cell's own machinery to make many copies of itself, and then destroying the cell as the new viruses are released, killing host cells and producing disease.

Transmission

Pathogens spread by routes such as droplets in the air, contaminated food or water, direct contact, body fluids, and vectors (such as insects). Understanding the route is the basis of controlling spread.

Examples in context

Example 1. Bacterial toxins. Some bacteria cause disease mainly through powerful toxins that damage tissues or block nerve or muscle function, even where the bacteria themselves are localised. This shows that the harm from a bacterial infection is not only about the number of bacteria present.

Example 2. Viral host specificity. A virus can only infect cells that carry the receptor its surface proteins bind to, which is why many viruses infect only particular tissues or species. This specificity, set by the virus's surface proteins, explains patterns of infection and is also exploited by vaccines.

Try this

Q1. State one structural feature present in a bacterium but absent in a virus. [1 mark]

  • Cue. A cell wall (or ribosomes, or a cell surface membrane, or cytoplasm).

Q2. Explain why a virus cannot reproduce outside a host cell. [2 marks]

  • Cue. A virus has no ribosomes or metabolic machinery of its own, so it must use a host cell's machinery to copy its genetic material and make its proteins.

Q3. State two ways a pathogen can be transmitted from one host to another. [2 marks]

  • Cue. Any two of: airborne droplets, contaminated food or water, direct contact, body fluids, or an insect vector.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original5 marksCompare the structure of a bacterium and a virus, and explain how each type of pathogen causes harm to the host.
Show worked answer →

The answer should contrast structure and then mechanism of harm.

A bacterium is a living prokaryotic cell, with a cell surface membrane, a peptidoglycan cell wall, circular DNA in a nucleoid, and 70S ribosomes, and it can reproduce by itself. A virus is not a cell: it is a particle consisting of genetic material (DNA or RNA) enclosed in a protein coat (capsid), sometimes with a lipid envelope, and it cannot reproduce without a host cell.

A bacterium causes harm by multiplying in the host and often by releasing toxins that damage cells or interfere with their function; it may also directly damage tissues. A virus causes harm by entering a host cell, using the cell's machinery to make many copies of itself, and then destroying the cell when the new viruses are released, killing host cells and causing disease.

Markers reward the cellular versus non-cellular distinction with correct structures, the bacterial mechanism (multiplication and toxins), and the viral mechanism (hijacking host cells to replicate and destroying them).

Original3 marksExplain why antibiotics are effective against bacterial infections but not against viral infections.
Show worked answer →

The answer turns on what antibiotics target.

Antibiotics work by targeting structures or processes specific to bacterial cells, such as the synthesis of the peptidoglycan cell wall, the function of the 70S ribosome, or bacterial enzymes. These targets exist in bacteria but not in the host's own cells, giving selective toxicity.

Viruses are not cells and lack these structures; they have no cell wall, no ribosomes of their own, and they replicate using the host cell's machinery. Antibiotics therefore have nothing virus-specific to act on, so they do not affect viruses.

Markers reward antibiotics targeting bacterial-specific structures or processes, the absence of these in viruses, and the conclusion that antibiotics cannot act on viruses.

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