Infectious Diseases in Humans
SEAB syllabus topic 9: infectious and non-infectious diseases, bacteria and viruses, influenza and pneumococcal disease, vaccines, antibiotics and antibiotic resistance, with the why, the traps and the exam checks.
Infectious diseases are caused by pathogens, and pathogens are passed from person to person through body fluids, food and water.
Vaccines stimulate white blood cells to make antibodies quickly when the pathogen invades.
Antibiotics act against bacteria, but they are ineffective against viruses.
Infectious and non-infectious diseases
state that infectious diseases can be spread from person to person whereas non-infectious diseases cannot and identify examples of each
An infectious disease can be spread from person to person, whereas a non-infectious disease cannot be spread from person to person.
- Disease
- Disease: a condition that stops the body, or a part of it, from working as well as it should, and that produces particular signs and symptoms
- Infectious disease
- Infectious disease: a disease that is caused by a pathogen and can be passed from one person to another
- Non-infectious disease
- Non-infectious disease: a disease that is not caused by a pathogen, so it cannot be passed from one person to another
- Communicable
- Communicable: another word for infectious; a non-infectious disease is also called non-communicable supports K325-9(a)
Infectious diseases and examples
Influenza
Influenza is an infectious disease of the airways that is passed from person to person in droplets from coughs and sneezes.
Pneumococcal disease
Pneumococcal disease is an infectious disease caused by a bacterium called pneumococcus.
Cholera
Cholera is an infectious disease caused by bacteria, and it is passed on through contaminated water and food.
HIV infection
HIV infection is an infectious disease caused by a virus, and the virus is passed on through body fluids such as blood.
Non-infectious diseases and examples
Atherosclerosis
In atherosclerosis, fatty deposits build up in the walls of the arteries and make them narrower, and the condition develops inside the person's own body.
Coronary artery disease
Coronary artery disease is a condition in which the coronary arteries, which supply the heart muscle with blood, become narrowed or blocked.
Cirrhosis
Cirrhosis is scarring of the liver, and one cause is long-term heavy alcohol consumption (some liver infections, such as hepatitis B or C, can also cause it); the scarred liver cannot be passed to another person.
Diabetes
In diabetes, the blood glucose concentration cannot be controlled properly, because the body makes too little insulin or the body cells do not respond to it.
Sickle-cell anaemia
Sickle-cell anaemia is an inherited disease in which faulty genes make abnormal haemoglobin, so the red blood cells are misshapen and carry less oxygen.
Scurvy
Scurvy is a deficiency disease, caused by too little vitamin C in the diet, so it is non-infectious. supports K325-9(a)
Infectious or non-infectious?
| Feature | Infectious disease | Non-infectious disease |
|---|---|---|
| Cause | An infectious disease is caused by a pathogen, such as a bacterium or a virus. | A non-infectious disease is caused by something other than a pathogen, such as faulty genes, a poor diet or damage to an organ. |
| Spread from person to person | An infectious disease can be passed from an infected person to another person. | A non-infectious disease cannot be passed from one person to another. |
| Examples | Examples of infectious diseases are influenza, pneumococcal disease, cholera and HIV infection. | Examples of non-infectious diseases are atherosclerosis, coronary artery disease, cirrhosis, diabetes and sickle-cell anaemia. |
Depth: why and how
- Why can a person not catch atherosclerosis from someone else? No pathogen is involved, so nothing that causes the disease can pass from one person to another.
- Why is sickle-cell anaemia not infectious even though it can run in a family? It is inherited through genes from the parents, which is not the same as being infected by a pathogen.
- How can a non-infectious disease be common in one family? Family members may share the same inherited genes, diet and habits, so they can develop the same disease without passing on a pathogen.
- Cross-link to Nutrition in humans: a deficiency disease such as scurvy is non-infectious, because it is caused by a lack of a nutrient in the diet.
- Cross-link to Transport in humans: coronary artery disease affects the coronary arteries, which supply the heart muscle with blood.
Common mistakes
All diseases can be caught from other people.
Infectious diseases can be passed from person to person, but non-infectious diseases such as diabetes cannot.
A disease that runs in a family must be infectious.
A disease that runs in a family may be inherited, as sickle-cell anaemia is, and inherited diseases are non-infectious.
Cirrhosis is infectious because it can be found in people who live together.
Cirrhosis is not passed from one person to another, and one cause is long-term heavy alcohol consumption.
Infectious diseases are caused by pathogens and can be spread from person to person, while non-infectious diseases such as diabetes and sickle-cell anaemia cannot.
Exam check
Q1 Which of the following is a non-infectious disease?
- A Influenza
- B Cholera
- C Sickle-cell anaemia
- D Pneumococcal disease
Show answer
MCQ answer key C (Sickle-cell anaemia): sickle-cell anaemia is inherited through faulty genes and is not caused by a pathogen, so it cannot be passed from person to person.
Q2 Which statement about infectious diseases is correct?
- A They are caused by pathogens and can be passed from person to person
- B They are caused by faulty genes passed from parents to children
- C They cannot be passed from one person to another
- D They are caused by a lack of a nutrient in the diet
Show answer
MCQ answer key A (They are caused by pathogens and can be passed from person to person): an infectious disease is caused by a pathogen, and the pathogen can be passed from an infected person to another person.
Q3 A student says, 'Atherosclerosis is infectious because several people in one family have it.' What is wrong with this statement?
- A It is caused by a virus that has not yet been found
- B It can be passed on in droplets
- C It is caused by bacteria in the blood
- D It is not caused by a pathogen, so it cannot be passed from one person to another
Show answer
MCQ answer key D (It is not caused by a pathogen, so it cannot be passed from one person to another): atherosclerosis develops inside the body when fatty deposits narrow the arteries, and no pathogen is involved.
Structured Influenza, diabetes, cholera and coronary artery disease are four diseases.
- State which two of the four diseases are infectious. [2]
- State one difference between an infectious disease and a non-infectious disease. [1]
- Explain why diabetes cannot be passed from one person to another. [2]
Total: 5 marks
Show mark scheme
(a) 2 marks
- Mark scheme: influenza
- Mark scheme: cholera
(b) 1 mark
- Mark scheme: an infectious disease is caused by a pathogen and can be spread from person to person, whereas a non-infectious disease cannot be spread from person to person
(c) 2 marks
- Mark scheme: it is not caused by a pathogen
- Mark scheme: so nothing that causes the disease can be transferred from one person to another
Pathogens and how they spread
explain that infectious diseases are caused by pathogens such as bacteria and viruses and can be spread from person to person through body fluids, food and water
Infectious diseases are caused by pathogens such as bacteria and viruses, and the pathogens are passed from person to person through body fluids, food and water.
- Pathogen
- Pathogen: a microorganism, such as a bacterium or a virus, that causes disease
- Transmission
- Transmission: the passing of a pathogen from an infected person to another person
- Body fluids
- Body fluids: liquids made by the body, such as blood, semen, vaginal fluid and breast milk
- Droplets
- Droplets: tiny drops of liquid from the nose and mouth that carry pathogens and are released when a person coughs, sneezes or talks
- Contaminated
- Contaminated: carrying pathogens; contaminated food, water and surfaces can pass pathogens to a person
Routes of transmission
Droplets
When an infected person coughs or sneezes, droplets that contain pathogens are released into the air, and another person can breathe them in through the nose or mouth into the respiratory tract.
Body fluids
Some pathogens are passed on in body fluids during close contact, for example HIV is passed on in the body fluids exchanged in sexual intercourse, and it is also passed on in blood.
Mother to baby
A mother who is infected can pass some pathogens to her baby in breast milk during breastfeeding.
Contaminated surfaces
Pathogens left on a surface such as a door handle can be picked up on the hands, and they enter the body when the person touches the nose, eyes or mouth.
Contaminated food
Food is contaminated when pathogens land on it or grow in it, and a person who eats the food can become ill, as in food poisoning.
Contaminated water
Cholera is a disease caused by bacteria, and it is spread when people drink water that is contaminated with the bacteria.
From one person to the next
- 1An infected person has pathogens in the body, for example in the nose, in the digestive system or in the blood.
- 2The pathogens leave the infected person, for example in droplets, in body fluids or in faeces.
- 3The pathogens travel to another person, in the air, on a surface, in food or water, or by direct contact.
- 4The pathogens enter the new person through the nose, mouth, eyes or broken skin, or by contact with body fluids, and the person may become ill.
Depth: why and how
- Why do pathogens spread more easily where many people live close together? Droplets and contaminated surfaces pass pathogens from person to person more often when people are near each other.
- Why is it important to wash the hands before eating? Washing removes pathogens picked up from surfaces, so they are not carried into the mouth with the food.
- Why can a person pass on a pathogen before feeling ill? Some pathogens multiply in the body for a while before symptoms appear, so an infected person can spread them without knowing.
- Cross-link to Nutrition in humans: the acid in the stomach kills many of the pathogens that are swallowed with food and drink.
- Cross-link to Respiration in humans: mucus and cilia in the airways trap pathogens that are breathed in and sweep them up towards the throat.
- Some infectious diseases, such as dengue, are carried from one person to another by a vector, which is an animal such as a mosquito. supports K325-9(b)
- Bacteria and viruses are not the only pathogens, because some are fungi, such as the fungus that causes athlete's foot, and some are protozoa, such as the parasite that causes malaria. supports K325-9(b)
Common mistakes
A pathogen is a disease.
A pathogen is the microorganism that causes a disease, and the disease is the condition that the pathogen produces.
Cholera is spread by droplets when an infected person coughs.
Cholera is spread through water and food that are contaminated with the bacteria.
Food and water that look clean cannot carry pathogens.
Food and water can carry pathogens even when they look clean, so water should be treated and food should be cooked and stored properly.
Dirty Boots Spread Filthy Water: Droplets, Body fluids, Surfaces, Food, Water.
D is for droplets, B for body fluids, S for surfaces, F for food and W for water; food and water can carry pathogens even when they look clean, so the hook words do not mean that only visibly dirty things spread disease.Pathogens are passed from person to person in droplets, body fluids, contaminated surfaces, food and water.
Exam check
Q1 Which of the following is a pathogen?
- A Cholera
- B Influenza virus
- C Diabetes
- D A droplet
Show answer
MCQ answer key B (Influenza virus): the influenza virus is a microorganism that causes disease, whereas cholera and diabetes are diseases and a droplet is only a carrier.
Q2 How is cholera spread?
- A By breathing in droplets from an infected person's cough
- B By a bite from an infected mosquito
- C Through sexual contact
- D By drinking water that is contaminated with the bacteria
Show answer
MCQ answer key D (By drinking water that is contaminated with the bacteria): cholera bacteria are passed on in contaminated water and food.
Q3 Which action helps to stop pathogens on a surface from entering the body?
- A Washing the hands before eating
- B Breathing through the mouth
- C Drinking more water
- D Sleeping for longer
Show answer
MCQ answer key A (Washing the hands before eating): washing removes pathogens from the hands, so they are not carried to the mouth, nose or eyes.
Structured Cholera and influenza are both infectious diseases.
- Name the type of organism that causes cholera. [1]
- State two ways in which pathogens can be passed from one person to another. [2]
- Explain why contaminated food can cause disease. [2]
Total: 5 marks
Show mark scheme
(a) 1 mark
- Mark scheme: bacteria
(b) 2 marks
- Mark scheme: any two of: droplets from coughs and sneezes, body fluids, contaminated surfaces, contaminated food, contaminated water
(c) 2 marks
- Mark scheme: the food carries pathogens
- Mark scheme: the pathogens enter the body when the food is eaten
Viruses
state that a typical virus has a protein coat enclosing its genetic material and reproduces only in living host cells
A typical virus has a protein coat that encloses its genetic material, and it reproduces only inside living host cells.
- Virus
- Virus: a very small pathogen that is not made of cells; it has a protein coat around its genetic material
- Protein coat
- Protein coat: the outer layer of a virus, made of protein, which encloses and protects the genetic material
- Genetic material
- Genetic material: the nucleic acid, DNA or RNA, that carries the instructions for making new viruses
- Spike protein
- Spike protein: a protein that sticks out from the surface of the protein coat of some viruses, such as the influenza virus
- Host cell
- Host cell: a living cell that a virus enters and uses to make new copies of itself
What a virus is made of
Structure
A virus consists of a protein coat that encloses the genetic material, and some viruses also have spike proteins on the outside.
Genetic material
The genetic material of a virus is either DNA or RNA, and the influenza virus has RNA.
No cell parts
A virus has no cell membrane, no cytoplasm and no organelles, so it is not a cell.
No growth or respiration
A virus does not grow and does not carry out respiration, because it has no cell structure.
Reproduction
A virus reproduces only inside a living host cell, by using the enzymes and ribosomes of that cell.
Inside a host cell: how a virus reproduces
- 1The virus attaches to the surface of a host cell, and the spike proteins help it to attach.
- 2The genetic material of the virus enters the host cell.
- 3The host cell uses its own enzymes and ribosomes to make new copies of the viral genetic material and new protein coats.
- 4The new genetic material and protein coats are put together to form many new viruses.
- 5The new viruses leave the host cell, which is often damaged or killed, and they infect other cells.
Depth: why and how
- Why can a virus not reproduce on its own? It has no ribosomes or cell machinery of its own, so it must use the ribosomes and enzymes of a living host cell.
- Why does a virus not grow? New viruses are put together at full size inside the host cell, so a single virus does not get bigger.
- Why does influenza affect the airways? The spike proteins of the influenza virus attach to cells that line the respiratory tract, so those cells are infected.
- Cross-link to Cell structure and organisation: a virus has no cell membrane, cytoplasm or organelles, so it is not a cell like a plant cell or an animal cell.
- Cross-link to Infectious and non-infectious diseases: the influenza virus is a pathogen, and it causes an infectious disease.
- The protein coat of a virus is also called the capsid. supports K325-9(c)
Common mistakes
A virus is a very small bacterium.
A virus is not a bacterium, because it has no cell structure and it is usually much smaller than a bacterium.
A virus has a cell membrane and cytoplasm like a bacterium.
A virus has a protein coat around its genetic material and it has no cell membrane, no cytoplasm and no ribosomes.
A virus reproduces by dividing into two.
A virus reproduces only inside a living host cell, by using the enzymes and ribosomes of the host cell.
A virus is a protein coat around DNA or RNA, and it reproduces only inside living host cells.
Exam check
Q1 Which structure encloses the genetic material of a virus?
- A Cell wall
- B Cell membrane
- C Protein coat
- D Cytoplasm
Show answer
MCQ answer key C (Protein coat): a virus has a protein coat that encloses its genetic material, and it has no cell wall, cell membrane or cytoplasm.
Q2 Where can a virus reproduce?
- A In any warm, moist place
- B Inside a living host cell
- C In a nutrient broth
- D On dead tissue
Show answer
MCQ answer key B (Inside a living host cell): a virus has no ribosomes or cell machinery of its own, so it can reproduce only inside a living host cell.
Q3 Which statement about a virus is correct?
- A It contains DNA or RNA inside a protein coat
- B It has a cell wall and a cell membrane
- C It grows by taking in nutrients
- D It has ribosomes in its cytoplasm
Show answer
MCQ answer key A (It contains DNA or RNA inside a protein coat): a virus consists of DNA or RNA inside a protein coat, and it does not grow or have a cell wall, cell membrane or ribosomes.
Structured A virus has been found in a specimen of mucus from a patient.
- State what encloses the genetic material of a virus. [1]
- State the two types of genetic material that a virus may contain. [1]
- Explain why a virus can reproduce only inside a host cell. [2]
Total: 4 marks
Show mark scheme
(a) 1 mark
- Mark scheme: the protein coat
(b) 1 mark
- Mark scheme: DNA and RNA (a virus contains one or the other)
(c) 2 marks
- Mark scheme: a virus has no ribosomes or cell machinery of its own (accept: cannot make new viruses without a host cell)
- Mark scheme: so it uses the enzymes and ribosomes of the host cell to make new viruses
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- Bacteria
- Signs and symptoms of influenza and pneumococcal disease
- Transmission and reducing the spread
- Vaccines
- Antibiotics
- Antibiotic resistance
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Notes version 1.0 · Last updated 30 Sept 2026 · Topic 9 of 14