Ch 1 · 10.1 Infectious diseases · AS & A Level Biology 9700

Topic 1 of 2 in Infectious diseases — AS & A Level Biology 9700 — 58 lessons.

Introduction

Infectious diseases are caused by pathogens that can be transmitted from one host to another, and their control depends on understanding how each pathogen is transmitted, how it affects the body, and the wider biological, social and economic context in which it spreads. The syllabus names four diseases: cholera, malaria, TB and HIV/AIDS. (Syllabus section 10.1.)

Key ideas — Infectious and non-infectious disease

An infectious disease is caused by a pathogen, a disease-causing organism or virus, that can be transmitted from an infected host to an uninfected host, directly or indirectly. A non-infectious disease, such as a genetic disorder or a disease caused by lifestyle factors, is not caused by a pathogen and cannot be passed between individuals in this way. The distinction matters because only infectious diseases can be controlled by interrupting transmission.

🧑‍🏫 Teacher's explanation

Think of an infectious disease like a viral video that spreads through social media; if one person shares it, others see it, and it keeps moving through the network. In biology, this 'sharing' is transmission. An infectious disease is caused by a 'pathogen'—a biological invader that wants to use your body to replicate.

📖 Definitions

🔍 Worked example — Easy

Identify the type of pathogen responsible for the following diseases: (i) Cholera, (ii) Malaria, (iii) Tuberculosis (TB).

🔍 Worked example — Standard

A patient presents with symptoms of Tuberculosis (TB). State the specific pathogen responsible and describe the mode of transmission.

🔍 Worked example — Exam-style

Compare the pathogens responsible for Malaria and HIV/AIDS in terms of their biological classification and transmission method.

⚠️ Common errors

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🎯 Exam question

Key ideas — The four named diseases

🧑‍🏫 Teacher's explanation

Think of an infectious disease like a computer virus. A virus is a piece of malicious code that needs a host to replicate and a way to spread to another computer. In biology, we call these 'pathogens'. An infectious disease is any disease caused by a pathogen that can be passed from one host to another (transmissible).\n\nTo understand how these diseases spread, you have to look at the 'mode of transmission'. Some pathogens travel through the air (like TB), some through contaminated water (like cholera), some through a middleman like a mosquito (like malaria), and some through direct contact with bodily fluids (like HIV). Understanding the pathogen type—whether it is a bacterium, a protoctist, or a virus—is the first step in learning how to stop them.

📖 Definitions

🔍 Worked example — Easy

Identify the pathogen type and specific name for the organism causing Malaria.

🔍 Worked example — Standard

Compare the transmission routes of Cholera and Tuberculosis (TB).

🔍 Worked example — Exam-style

A patient presents with symptoms of TB. Evaluate the potential sources of infection for this patient, distinguishing between the two main bacterial causes.

⚠️ Common errors

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🎯 Exam question

Key ideas — Effects on the human body

(Background only: the syllabus examines transmission and control, not symptoms.) Cholera toxin causes chloride ion secretion into the gut lumen, drawing water out by osmosis; this produces severe watery diarrhoea and can lead to fatal dehydration and ion imbalance. Malaria parasites multiply in the liver then invade red blood cells, which rupture in synchronised cycles, causing recurring fever, anaemia and, in severe cases, blockage of blood vessels in the brain, cerebral malaria.

🧑‍🏫 Teacher's explanation

Think of an infectious disease like a security breach in a high-security building. A pathogen is an intruder that enters the building (the body) and starts causing chaos. Sometimes the intruder is a single person (a bacterium), sometimes it is a complex group (a protoctist), and sometimes it is a tiny, invisible virus. The damage isn't just caused by the intruder itself, but often by the body's own security system overreacting or being overwhelmed.

📖 Definitions

🔍 Worked example — Easy

Complete the following table for the pathogens associated with the specified diseases:

🔍 Worked example — Standard

Explain the physiological mechanism by which cholera leads to severe dehydration.

🔍 Worked example — Exam-style

Compare the impact of Mycobacterium tuberculosis and HIV on the human immune system and respiratory system.

⚠️ Common errors

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🎯 Exam question

(a) Identify the pathogen responsible for this disease. [1] (b) Describe the effect this disease has on the body's ability to resist infection. [2] [3 marks]

Key ideas — Biological, social and economic factors in control

Biological factors include the availability of an effective vaccine or drug treatment, the existence of an animal reservoir or insect vector, and the rate at which a pathogen mutates or develops drug resistance. Social factors include population density and overcrowding, sanitation and access to clean water, cultural or religious practices, and public understanding of how a disease spreads. Economic factors include the cost of vaccination, drug and vector-control programmes, and a country's ability to fund healthcare infrastructure, clean water supplies and health education, which are often limited in lower-income countries where several of these diseases are most prevalent.

🧑‍🏫 Teacher's explanation

Think of an infectious disease like a wildfire. To stop a fire, you need to understand the biology (the type of fuel and how fast it burns), the social environment (how close the houses are and if people use campfires), and the economics (whether the town can afford a fire brigade and water tanks).

📖 Definitions

🔍 Worked example — Easy

Complete the following table identifying the pathogen and type for the specified diseases:

🔍 Worked example — Standard

A researcher is studying the transmission of Malaria. Explain why the presence of stagnant water in a village is considered a social/environmental factor in the control of the disease.

🔍 Worked example — Exam-style

Discuss the biological and economic factors that make the control of HIV/AIDS particularly challenging compared to a bacterial infection like Cholera.

⚠️ Common errors

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🎯 Exam question

(a) State the name and type of pathogen that causes cholera. [2] (b) Discuss the biological, social, and economic factors that must be considered to control the spread of this disease. [4] [6 marks]

Key ideas — Methods of reducing transmission

Transmission can be reduced by improving sanitation and the provision of clean drinking water, which targets cholera; using insecticide-treated bed nets, indoor spraying and eliminating mosquito breeding sites, vector control against malaria; vaccination, such as BCG for tuberculosis and oral vaccines for cholera; health education promoting safe sex, needle exchange schemes and screening of blood for transfusion against HIV; and prompt diagnosis, isolation of infectious individuals and completion of full courses of drug treatment for tuberculosis and cholera.

🧑‍🏫 Teacher's explanation

Think of an infectious disease like a wildfire. To stop a wildfire, you can either remove the fuel (sanitation/clean water), create firebreaks to stop the spread (bed nets/insecticide), or train people to use fire extinguishers before the fire gets too big (vaccination/education). In biology, we call the 'fire' a pathogen. To stop it, we must understand how it moves from one host to another.

Ch 1 · 10.1 Infectious diseases · AS & A Level Biology 9700

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