10.1.210-11 · Infectious Disease and Immunity

Transmission of Disease — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 12 parts Lesson 3 of 21 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision

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10.1.2 Transmission of Disease — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe how the pathogens of common diseases are transmitted and the factors that affect transmission.Definition — transmission is the passing of a pathogen from an infected host to a ne…

Direct contact
Body fluids
Droplet infection
Placenta and birth
Contaminated food and water
Overcrowding

Part 1 of 10Direct transmission

Definition

Definition transmission is the passing of a pathogen from an infected host to a new host,
- and it is direct when no intermediate object or organism is involved.

Direct contact

Direct contact touching or skin contact, as with some skin fungi.

Body fluids

HIV passes via blood, semen, vaginal fluids and breast milk; sharing needles transfers blood.

Droplet infection

Droplet infection coughing, sneezing or talking releases droplets carrying pathogens such as TB, measles and influenza, which another person inhales.

Placenta and birth

Placenta and birth pathogens can pass across the placenta or during birth.

Part 2 of 10Indirect transmission

Indirect transmission

Contaminated food and water

Contaminated food and water cholera and typhoid.

Fomites

Fomites objects such as towels, bedding or surgical instruments.

Vectors

Vectors organisms that carry the pathogen, such as the Anopheles mosquito carrying Plasmodium.

Part 3 of 10Individual diseases

Cholera

Vibrio cholerae is egested in faeces, so water becomes contaminated.
Infection follows from washing in, drinking or eating food exposed to this water.
It occurs where sanitation and clean food are lacking.

Malaria

Plasmodium is spread by an insect vector.
Female Anopheles feed on blood to get protein for their eggs.
The mosquito takes up the pathogen from an infected person and passes it to the next person it bites.
It can also spread by blood transfusion, reused unsterile needles and across the placenta.

Mycobacterium tuberculosis spreads by droplet infection from people with active disease, so it spreads faster in overcrowded conditions.
The bovine form (M. bovis) passes from cattle via contaminated meat or unpasteurised milk.

HIV

HIV it passes only by direct exchange of body fluids.
Routessexual intercourse, blood donation, shared needles, across the placenta, mixing of blood at birth and breast milk.

Part 4 of 10Examples from the specification

Disease	Transmission	Notes
Cholera	Faeces-contaminated water, food	Spread by flood or poor sanitation
Malaria	Bite of female Anopheles	Mosquito takes blood meal, passes the parasite in saliva
TB	Inhaling droplets; infected milk (M. bovis)	Spread in crowded, badly ventilated places
HIV/AIDS	Unprotected sex, blood, placenta, breast milk	Fragile virus does not survive outside the body
Measles	Airborne droplets	One of the most contagious diseases
Smallpox	Droplets, direct contact, contaminated bedding	Only human host, aided eradication
BSE	Cattle fed meat-and-bone meal; humans eat infected nervous tissue	Prions resist cooking

Part 5 of 10Symptoms of the four main diseases

Disease	Pathogen	Symptoms
Cholera	Vibrio cholerae	Diarrhoea and dehydration
Malaria	Plasmodium falciparum, P. malariae, P. ovale, P. vivax	Fever, headaches and muscle pain
TB	Mycobacterium tuberculosis, M. bovis	Cough, chest pain, fever and weight loss
HIV/AIDS	Human immunodeficiency virus	Initial flu-like symptoms, then loss of immune function (AIDS)

Part 6 of 10Factors affecting transmission

1Overcrowding

Overcrowding close contact increases droplet and contact transmission.

2Poor nutrition and underlying illness

Poor nutrition and underlying illness they weaken the immune system, raising susceptibility.
TB and HIV are a deadly combination because HIV weakens defences.

3Climate

Climate warm, wet climates favour mosquito breeding, and seasonal flooding spreads cholera.

4Poor sanitation

Poor sanitation lack of clean water, particularly in refugee camps and after natural disasters, spreads disease.

5Culture and behaviour

Culture and behaviour unprotected sex, needle sharing and burial practices involving contact with the dead.

6Healthcare access

Healthcare access lack of vaccination, diagnosis and treatment, and poor hospital hygiene.

7Travel and migration

Travel and migration infected travellers carry pathogens across the world rapidly.

8Pathogen traits

Pathogen traits survival time outside the host, infectious dose and antigenic variation.

Part 7 of 10Chain of transmission

Links

Links a reservoir of the pathogen, a way of leaving the host, a route of transmission, a way of entering a new host and a susceptible host.

Breaking the chain

Breaking the chain removing any link stops the spread.
Malariadrain stagnant water, use bed nets and apply insecticide to act on the vector.
Measlesvaccination removes the susceptible host.
Cholerawater treatment removes the route.

Properties and routes

HIV is a fragile enveloped virus quickly inactivated outside the body, so it needs fluid exchange.
TB bacteria have a waxy cell wall and survive in dried droplets, so airborne spread is possible.

Carriers

Carriers asymptomatic carriers are important in the spread of some diseases.

Part 8 of 10Measuring disease

Measuring disease

Incidence

Incidence new cases per year per 100 000 people.

Prevalence

Prevalence total cases at a time.

Basic reproduction number (R0)

Basic reproduction number (R0) the average number of people infected by one case in a susceptible population.
When R₀ is above 1 an outbreak grows.
Control measures aim to push it below 1.

Part 9 of 10More on this topic

Transmission
- The amount of light passing through a sample.

Part 10 of 10More from the specification

Pharmacological drugs and pathogen vectors
- administering prophylactic drugs provides an effective means to halt pathogen transmission across vulnerable human populations.

Key terms

Term	Meaning
Direct transmission	Passing of a pathogen without an intermediate object or organism.
Indirect transmission	Passing of a pathogen via an intermediate such as food, a fomite or a vector.
Fomite	An inanimate object that carries a pathogen.
Droplet infection	Spread by droplets released in coughing, sneezing or talking.
Reservoir	The place where a pathogen persists.
R_{0}	The average number of people infected by one case in a susceptible population.

Avoid these mistakes

Saying HIV spreads through air or casual contact — it needs direct exchange of body fluids.

Saying a vector is the pathogen — a vector carries the pathogen between hosts.

Saying TB spreads by contaminated water — it spreads by droplets (and M. bovis via milk).

Saying only one route exists for each disease — some pathogens use more than one.

Last updated: 15 October 2026