10.110-11 · Infectious Disease and Immunity

Infectious Diseases — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

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

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10.1 Infectious Diseases — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe the causes, transmission, control and global patterns of cholera, malaria, TB, HIV/AIDS, measles and smallpox.Infectious disease — a disease caused by a pathogen that can pass between …

Non-infectious disease
Prevalence
Mortality rate
Attachment
Toxin action
Vaccination

Part 1 of 11Infectious and non-infectious disease

Infectious disease

Infectious disease a disease caused by a pathogen that can pass between hosts.

Non-infectious disease

Non-infectious disease it is not caused by a pathogen and may be inherited or related to lifestyle.

Incidence

Measures of disease.

Incidence number of new cases in a period per population.
Prevalence total number of cases at a given time per population.
Mortality rate deaths from the disease ÷ population (often per 100 000 per year).

Part 2 of 11Cholera

Infection

Vibrio cholerae is ingested in contaminated water;
- most are killed by stomach acid, but survivors reach the small intestine.

Attachment

Attachment they attach to the epithelium with flagella and release cholera toxin.

Toxin action

Toxin action it opens Cl^- channels so Cl^- leaves the epithelial cells.
Water potential of the lumen falls and water follows by osmosis, causing diarrhoea and dehydration.

Treatment

Treatment oral rehydration therapy.
- (glucose plus salts)

Vaccination

Vaccination oral cholera vaccines are used where the disease is endemic or in outbreaks alongside clean water and sanitation.
- (for example refugee camps)
Antibiotics are added only for severe cases.

Part 3 of 11Malaria

1Vector

Vector only female Anopheles mosquitoes transmit malaria because only they take blood meals.

2Stages

Plasmodium has stages in the mosquito gut and salivary glands, human liver cells and red blood cells.

Why control is hard.

Plasmodium is eukaryotic and changes its antigens.
Mosquitoes develop insecticide resistance and parasites develop drug resistance.
The parasite lives inside host cells, hidden from antibodies.

41950s eradication campaign

1950s eradication campaign the WHO programme failed overall because Plasmodium became resistant to drugs and Anopheles to DDT and other insecticides.

5Modern control

Modern control faster diagnosis, better drug supply, combination therapy to slow resistance, treated nets and biological control of larvae.
Rapid dipstick tests need no laboratory.
The Plasmodium genome has been sequenced.
Malaria vaccines (RTS,S and R21) have been approved.

Part 4 of 11Malaria transmission cycle

An infected female Anopheles takes a blood meal and injects Plasmodium in saliva.

Parasites travel in the blood to the liver and multiply in liver cells.

Parasites invade red blood cells, multiply and burst them, causing fever.

Some parasites become gametes in the blood.

An uninfected mosquito ingests gametes in a blood meal, and fertilisation occurs in its gut.

Infective stages move to the salivary glands, ready to be passed on.

Part 5 of 11Tuberculosis

Transmission

Transmission droplets are inhaled; bacteria are taken up by phagocytes in the lungs but survive inside them.

Dormancy

TB bacteria can stay dormant in the lungs for years and become active when immunity is weakened, for example by HIV.

Active TB

Active TB it destroys lung tissue, causing coughing (with blood), weight loss and fever.

Why control is hard.

Treatment lasts 6 to 9 months, which leads to non-compliance and multidrug-resistant strains.
The BCG vaccine gives variable protection.
TB is linked with HIV and overcrowding.

Part 6 of 11HIV/AIDS

Retrovirus

HIV has an RNA genome, reverse transcriptase, a protein capsid and a lipid envelope with glycoproteins.

Infection

Infection it binds CD4 on helper T-cells and macrophages and uses reverse transcriptase to make DNA, which is inserted into host DNA.
New copies bud off the cell.

AIDS

AIDS it develops when helper T-cells fall so low that B-cells and killer T-cells are not activated.
Opportunistic infections (TB, pneumonia, Kaposi sarcoma) then kill the patient.

Treatment

Treatment antiretroviral drugs slow replication but do not cure.
- (for example reverse transcriptase inhibitors)

No vaccine

HIV mutates rapidly and hides inside cells.

Why control is hard

Why control is hard there is a long symptomless (latent) period,
so carriers may not know they are infected, and rapidly changing surface antigens hinder immune recognition.
Control therefore relies on testing, contact tracing and changed behaviour.

Breastfeeding advice

Breastfeeding advice it depends on the setting.
Where clean water is scarce, breastfeeding may be safer overall because formula made with dirty water risks diseases such as cholera.
Breast milk also gives antibodies.

Part 7 of 11Measles and smallpox

Measles

Measles a virus that infects the respiratory tract;
- complications include pneumonia and encephalitis, and malnourished children are at high risk.
It is controlled by the MMR vaccine.
It is so infectious that about 95 per cent vaccination coverage is needed for herd immunity.

Smallpox

Smallpox the only human disease eradicated (1980), by a WHO ring-vaccination programme.

Part 8 of 11Global patterns and social factors

Global patterns and social factors

Cholera and malaria

Cholera and malaria they are concentrated in tropical, poorer regions such as sub-Saharan Africa and South Asia.

TB and HIV

TB and HIV they are linked with poverty, overcrowding and each other.

Social and economic factors

Social and economic factors poverty, war, refugee camps, poor sanitation, lack of healthcare, cost of drugs and nets,
- and education often matter as much as biology.

Part 9 of 11Stages of an infection

The pathogen leaves an infected host (faeces, droplets, blood, or via a vector).

It is transmitted to a new host (water, air, vector bite, body fluids).

It enters through a body surface (gut lining, alveoli, skin puncture, mucous membranes).

It multiplies in host cells or tissues.

Damage by toxins or cell destruction produces symptoms.

New pathogens are released and passed on, unless control measures break transmission.

Part 10 of 11More from the specification

1Antibiotic stewardship in healthcare

Antibiotic stewardship in healthcare prudent antimicrobial prescribing provides an effective clinical means to prevent selection of resistant microbial strains.

2Tuberculosis

Lung disease caused by Mycobacterium tuberculosis.

Part 11 of 11More on this topic

Other vaccinations

Other vaccinations adults and healthcare workers are offered TB and hepatitis B;
- travellers are offered yellow fever.

Saying stopping a course early is harmless — it leaves partly resistant survivors.

Key terms

Term	Meaning
Incidence	Number of new cases in a period per population.
Prevalence	Total number of cases at a given time per population.
Retrovirus	A virus with an RNA genome that uses reverse transcriptase to make DNA.
Reverse transcriptase	Enzyme that makes DNA from an RNA template.
Herd immunity	Protection of a population when enough people are immune.
Opportunistic infection	An infection that takes hold because the immune system is weakened.

Avoid these mistakes

Saying all mosquitoes transmit malaria — only female Anopheles do, because only they take blood meals.

Saying cholera toxin makes water leave because of the toxin directly — Cl^{-} secretion lowers lumen water potential and water follows by osmosis.

Saying antiretroviral drugs cure HIV — they only slow replication.

Confusing incidence with prevalence — incidence is new cases, prevalence is all cases at a time.

Last updated: 18 October 2026