Define infectious and transmissible disease
Explain how pathogens cause infectious disease and what it means for a disease to be transmissible.
Section 10.1 · Infectious diseases
Cambridge International AS & A Level Biology 9700, 2028–2030 syllabus, Version 1. Educational biology content only: it does not diagnose illness or replace advice from a qualified health professional.
Before you begin
- A pathogen is a disease-causing microorganism or infectious agent; transmission describes how it passes between hosts.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What causes an infectious disease?
What you will learn
- State that infectious diseases are caused by pathogens
- State that infectious diseases can be transmitted between hosts
Build the explanation
An infectious disease is caused by a pathogen, an infectious agent that can enter a host and cause disease. Infectious diseases are transmissible: the pathogen can pass from one host to another, directly or indirectly. Transmission may occur through contaminated food or water, air, body fluids or a vector. The route depends on the pathogen and the conditions that allow it to spread.
Apply it carefully
Do not use 'infectious' and 'contagious by casual contact' as synonyms. Some infections spread by a vector or contaminated environment rather than ordinary person-to-person touch. A person may carry or transmit a pathogen without obvious symptoms. Identify the specific route before suggesting a control method.
Pause and explain
What does transmissible mean?
Worked example
What makes a disease infectious?
Show the worked solution
- Identify the causal agent.
- An infectious disease is caused by a pathogen.
Answer It is caused by a pathogen that can enter a host and cause disease; infectious diseases can be transmitted between hosts.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Define pathogen and state what it means for a disease to be transmissible.
Give me a hint
Define the agent first, then the movement between hosts.
Compare my reasoning
- A pathogen is a disease-causing infectious agent.
- Transmissible means it can pass from one host to another.
A pathogen is an infectious agent that causes disease. A transmissible disease can pass from an infected host or source to another host, by a route appropriate to that pathogen.
Look for these in your work
- Pathogen defined
- Transmission between hosts stated
A disease spreads through contaminated water but not by touching an infected person. Can it still be transmissible? Explain.
Give me a hint
Transmission need not be direct contact.
Compare my reasoning
- Contaminated water can carry a pathogen between hosts.
- The disease is transmissible even without direct person-to-person touch.
Yes. A pathogen in contaminated water can pass between hosts indirectly, so the disease is transmissible even if ordinary touch is not the route.
Look for these in your work
- Indirect route recognised
- Term transmissible applied correctly
A person carries a pathogen but has no symptoms. Explain why this does not prove that transmission is impossible.
Give me a hint
Separate symptoms from presence and spread of the pathogen.
Compare my reasoning
- Symptoms describe effects in the host.
- A pathogen may still be present and could be transmitted by its route.
The absence of symptoms does not establish that the pathogen is absent. If the pathogen is present and can leave the host by a route such as body fluids or faeces, transmission may still occur; the route and evidence must be considered.
Look for these in your work
- Symptoms distinguished from infection
- Route-dependent transmission stated
- No unsupported certainty
Common mistakes
- Defining all transmissible disease as spread by touch
- Confusing a pathogen with a symptom
- Assuming no symptoms means no pathogen can be present
What can you explain now?
What causes an infectious disease?
Compare with the explanation
A pathogen
Pathogens are infectious agents that can cause disease in a host.
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Make it stick
How do infectious disease, pathogen and transmissible disease relate?
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