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Pathogens and disease

Compare pathogen types and describe how diseases spread.

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The key idea

Pathogens include bacteria, viruses, fungi and protists. Communicable diseases can spread directly or indirectly.

Four pathogen groupsTheir cell structures and reproduction differ.
Four pathogen groupsTheir cell structures and reproduction differ.bacteriumvirusfungal hyphaeprotistbacteria, viruses, fungi, protists
Revision notes

The bit that matters

Learn the process in clean chunks. If a sentence explains a cause, make sure you can say the effect too.

1

Types of pathogen

A pathogen is a microorganism that causes infectious disease, and they include bacteria, viruses, fungi and protists.Bacteria and viruses can reproduce rapidly inside the body.Bacteria may produce toxins that damage tissues and make us feel ill, whereas viruses live and reproduce inside cells, damaging them as new viruses are released.

2

How pathogens spread

Pathogens can spread through the air (droplet infection when someone coughs or sneezes), through direct contact, and through contaminated water or food.The spread of disease can be reduced by being hygienic, destroying vectors, isolating infected individuals and vaccination.Understanding how a disease spreads is essential for controlling outbreaks.

3

Examples of diseases

Measles is a viral disease spread by droplets that can be fatal if complications arise, and is now rare due to vaccination.HIV is a virus spread by sexual contact or exchange of body fluids that attacks the immune system and can lead to AIDS.Salmonella is a bacterium causing food poisoning, and the rose black spot is a fungal disease of plants.

4

Plant diseases and defences

Plants are attacked by pathogens such as tobacco mosaic virus, which causes a mosaic pattern of discolouration on leaves and reduces photosynthesis.Plants have physical defences such as a waxy cuticle and cell walls, chemical defences such as antibacterial substances and poisons, and mechanical defences such as thorns and hairs.Ion deficiencies, such as a lack of nitrate ions, can also stunt growth and look like disease.

Key terms

Definitions to learn

Pathogen

A microorganism that causes an infectious disease.

Communicable disease

A disease caused by a pathogen that can be passed from one organism to another.

Toxin

A poison produced by some bacteria that damages cells and makes us feel ill.

Vector

An organism that carries and transmits a pathogen from one host to another.

Droplet infection

The spread of a pathogen through tiny droplets released when coughing or sneezing.

Vaccination

Introducing a weakened or inactive form of a pathogen to give immunity to a disease.

Worked example

Explain why antibiotics cannot treat influenza.

1

Influenza is caused by a virus.

2

Viruses reproduce inside host cells.

3

Antibiotics target bacterial processes, not viruses.

Final answer

Antibiotics do not treat viral infections such as influenza.

Exam habit

Name the specific pathogen type: bacterium, virus, fungus or protist. 'Germs' and 'microbes' score no marks.Antibiotics treat bacteria only — never viruses.

Watch out

Do not call every microorganism a pathogen. A pathogen causes disease.

Examiner tips

How to score full marks

  • 1Bacteria make you ill mainly by producing TOXINS; viruses make you ill by reproducing inside and DAMAGING your cells — state the correct mechanism.
  • 2Name the TYPE of pathogen (bacterium / virus / fungus / protist) when asked about a specific disease — examiners award marks for the type, not just the name.
  • 3For 'reduce spread' questions, give specific methods (hygiene, isolation, vaccination, destroying vectors) rather than vague answers like 'be clean'.
Practice questions

Try these yourself

Open each answer only after you have explained the full biological process.

1Name the pathogen that causes malaria and its vector.
Mark scheme
  1. 1.Recall the pathogen type and carrier.
Malaria is caused by a protist and spread by mosquitoes.
2Give one way to reduce transmission of measles.
Mark scheme
  1. 1.Think about immunity or isolation.
Vaccination or isolating infected people.
3Why can bacterial infections sometimes be treated with antibiotics?
Mark scheme
  1. 1.Compare bacterial and human cell processes.
Some antibiotics disrupt bacterial processes without damaging human cells.
4Define the term pathogen.[1 mark]
Mark scheme
  1. 1.Recall the precise definition.
A microorganism that causes infectious / communicable disease (1).
5Name the type of pathogen that causes measles and one that causes salmonella food poisoning.[2 marks]
Mark scheme
  1. 1.Recall the pathogen type for each disease.
Measles is caused by a virus (1); salmonella food poisoning is caused by a bacterium (1).
6Describe two ways the spread of a communicable disease can be reduced.[2 marks]
Mark scheme
  1. 1.Give specific control methods.
Any two from: being hygienic / washing hands (1); isolating infected individuals (1); destroying vectors (1); vaccination (1).
7Explain why infection by some bacteria makes a person feel ill.[3 marks]
Mark scheme
  1. 1.Link the bacteria reproducing to the production of toxins.
The bacteria reproduce rapidly inside the body (1) and produce toxins / poisons (1) which damage cells and tissues, making the person feel ill (1).
8A field of tomato plants shows yellow mosaic patterns on the leaves and stunted growth. Explain how the gardener could decide whether this is caused by a pathogen or a mineral deficiency, and how each would reduce the yield.[6 marks]
Mark scheme
  1. 1.Consider the cause of the symptoms and the effect on photosynthesis or growth.
The mosaic pattern suggests tobacco mosaic virus, a pathogen (1), which destroys chloroplasts / reduces the area of leaf for photosynthesis (1), lowering yield. Alternatively a nitrate ion deficiency causes stunted growth (1) because nitrates are needed to make proteins for growth (1). The gardener could test the soil for mineral ions / look for spread to other plants to distinguish them (1); a pathogen tends to spread between plants whereas a deficiency would depend on soil conditions (1).
9Name the vector that transmits malaria and state the type of pathogen that causes the disease.[2 marks]
Mark scheme
  1. 1.Recall the organism that carries malaria and the pathogen type.
The mosquito is the vector (1); malaria is caused by a protist (1).
10Explain how viruses cause damage to the cells of a host organism.[3 marks]
Mark scheme
  1. 1.Describe how a virus reproduces inside a cell and what happens when new viruses are released.
A virus enters a host cell and uses its biochemistry to replicate (1); new virus particles are assembled inside the cell (1); the new viruses burst out of / are released from the cell, damaging or destroying it (1), which causes the symptoms of the disease.
11HIV infection can lead to AIDS. Explain why a person with AIDS is particularly vulnerable to other infections.[4 marks]
Mark scheme
  1. 1.Describe what HIV attacks.
  2. 2.Link this to the ability to fight other pathogens.
HIV attacks white blood cells / lymphocytes (1); this reduces the number of white blood cells and weakens the immune system (1); the body cannot produce enough antibodies to fight off other infections (1), so the person becomes ill from pathogens that a healthy immune system would normally destroy (1).
12A person returns from holiday with symptoms of a stomach infection caused by Salmonella bacteria. Explain three measures that would prevent the spread of the infection to other people in the household.[3 marks]
Mark scheme
  1. 1.Think about hygiene, contamination routes and isolation.
Any three from: washing hands thoroughly after using the toilet (1) to prevent faecal-oral transmission; cooking food thoroughly to kill bacteria (1); keeping the infected person's utensils and towels separate (1) to prevent direct contact spread; isolating the infected person from food preparation (1); disinfecting surfaces in the kitchen and bathroom (1).
13Describe two physical defences and one chemical defence that plants use to protect themselves against pathogens.[3 marks]
Mark scheme
  1. 1.Recall structural features and chemical compounds plants produce.
Physical: waxy cuticle on leaves (1) prevents pathogens entering the leaf surface; cell walls made of cellulose (1) provide a barrier against pathogens entering cells; bark on stems (1) prevents entry of pathogens; mechanical defences such as thorns or hairs (1). Chemical: antimicrobial substances / antibacterial chemicals (1) that kill pathogens; poisons / toxins to deter herbivores that might spread disease (1). (Any two physical and one chemical with brief explanation.)
14Evaluate the effectiveness of Ignaz Semmelweis's discovery that handwashing reduced deaths in hospitals, and explain why his ideas were not immediately accepted by other doctors.[5 marks]
Mark scheme
  1. 1.Describe what Semmelweis found.
  2. 2.Explain the benefit.
  3. 3.Explain why it was rejected.
Semmelweis showed that doctors washing hands in antiseptic solution before treating patients greatly reduced deaths from infection (1); this established the idea that disease can be transferred by contact / unhygienic practices (1); it was not accepted immediately because germ theory had not yet been established so doctors did not understand why handwashing worked (1); doctors were also reluctant to accept they themselves were spreading disease (1); eventually the link between pathogens and infection was proven by later scientists, confirming his approach (1).
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