O-Level and SEC G3 Biology K325
B9: Infectious Diseases in Humans
Compare bacteria and viruses, then connect influenza, pneumococcus, vaccination, antibiotics, and resistance.
Reviewed for the 2027 cohort on 19 July 2026. The official syllabus remains authoritative for assessable wording and paper details.
Core notes
Infectious Diseases in Humans links bacterial and viral structure to transmission, vaccination, antibiotics, and resistance. Keep prevention separate from treatment and explain why antibiotics affect susceptible bacteria but not viruses.
Bacteria, viruses and disease evidence
Infectious diseases can spread between people, while non-infectious diseases cannot. A typical bacterium has a cell wall and DNA not enclosed by a nucleus; some bacteria are pathogenic and many are not. A typical virus has genetic material inside a protein coat and reproduces only within living host cells.
Influenza is caused by influenza virus and commonly produces fever, body aches, tiredness, cough, and other respiratory symptoms. Pneumococcal disease is caused by pneumococcus bacteria and can produce fever, cough, breathing difficulty, or more severe illness. Use the case evidence rather than diagnosing from one symptom.
Transmission control and vaccination
Influenza virus and pneumococcus can spread in respiratory droplets and secretions. Measures can include vaccination, covering coughs, hand hygiene, suitable ventilation, reducing close contact when ill, and following current health guidance. Food, water, and body-fluid controls apply when those are the stated transmission routes.
Vaccines contain an agent resembling a pathogen and stimulate white blood cells to produce antibodies. A later invasion can trigger a faster antibody response. Vaccination reduces disease risk and can reduce spread, but no single control removes every transmission event.
Treatment, antibiotics and resistance
Antibiotics act on susceptible bacteria by disrupting bacterial structures or processes and do not treat viral infections. Viruses lack the same cellular structures and reproduce inside host cells, so an antibiotic target may be absent. Treatment decisions should follow current medical guidance.
Random mutation can give some bacteria resistance. Antibiotic exposure kills susceptible bacteria, resistant bacteria survive and reproduce, and the resistance allele becomes more common. Use antibiotics only when clinically appropriate and according to current medical instructions; do not rely on a universal slogan in place of professional advice.
Formulae and relationships
This chapter is assessed mainly through models, field patterns and explanations. Build the causal chain before adding any calculation.
Worked examples
Example 1: After an antibiotic is used repeatedly, a resistant bacterial strain becomes common. Explain this as natural selection.
- Variation exists because mutation produced a resistance allele before or during population growth.
- The antibiotic is a selection pressure that kills more susceptible bacteria.
- Resistant survivors reproduce and pass resistance to descendants, increasing its frequency.
Answer: Differential survival and reproduction make the resistance allele more common in the population.
Chapter checkpoint
Use these three moves to organise the topic before attempting a mixed or practical question.
- Compare typical bacterial and viral structure and distinguish infectious from non-infectious disease.
- Connect influenza and pneumococcal signs, transmission, vaccination, and control measures.
- Explain antibiotic selectivity and how misuse or overuse accelerates resistant bacteria.
Official outcome coverage
K325 B9: 9 mapped outcomes, references B9(a), B9(b), B9(c), B9(d), B9(e), B9(f), B9(g), B9(h), B9(i). Check the official K325 syllabus.
The outcome wording is not reproduced here. The relevant official syllabus remains authoritative for exact assessable scope.
Practical and data connection
Interpret microbial-growth or epidemiological data safely; school work should use approved simulations or non-pathogenic procedures only.
Exam traps and retrieval check
Avoid these traps
- Saying a person becomes antibiotic resistant instead of the bacterial population.
- Claiming antibiotics kill viruses.
- Naming a control measure without matching it to the transmission route.
Check from memory
Which cells produce antibodies?
Lymphocytes.
Why can a second immune response be faster?
Memory cells remain after the first response.
What creates new resistance alleles?
Random mutation, not the need of an individual bacterium.
Pure versus Combined scope
Combined Biology shares the core idea but assesses a narrower outcome set. Use the K327 or K328 component checklist to set the exact boundary.
Shared explanation source
Eclat has a related explanation in its existing IP library. It can help with the shared concept, but its IP extensions and school-sensitive scope are not automatically part of K325. Open the related IP explanation.