Microorganisms and Disease

What you'll learn

This revision guide covers how microorganisms cause disease and how the body defends itself against infection. You'll learn about different types of pathogens, how diseases spread, the body's natural and acquired immunity, and methods of preventing and treating infectious diseases—all aligned with the WJEC GCSE Biology specification.

Key terms and definitions

Pathogen — a microorganism that causes disease (includes bacteria, viruses, fungi and protists)

Communicable disease — an infectious disease that can be transmitted from one organism to another

Vector — an organism that transmits a pathogen from one host to another without being infected itself

Antibody — a protein produced by white blood cells that binds to specific antigens on pathogens

Antigen — a protein or carbohydrate marker on the surface of a pathogen that triggers an immune response

Phagocytosis — the process by which white blood cells engulf and digest pathogens

Vaccination — the administration of dead or weakened pathogens to provide immunity without causing disease

Antibiotic — a drug that kills bacteria or prevents their growth (does not work against viruses)

Core concepts

Types of pathogens

Pathogens are divided into four main groups, each with distinct characteristics that affect how they cause disease and how we treat infections.

Bacteria

• Single-celled prokaryotic organisms
• Reproduce rapidly by binary fission
• Produce toxins that damage tissues and cause disease symptoms
• Can be treated with antibiotics
• Examples: Salmonella (food poisoning), Mycobacterium tuberculosis (tuberculosis), Neisseria gonorrhoeae (gonorrhoea)

Viruses

• Non-living particles much smaller than bacteria
• Cannot reproduce independently—must invade host cells
• Take over cell machinery to replicate, then burst out and destroy the cell
• Cannot be treated with antibiotics
• Examples: influenza virus, HIV, measles virus, tobacco mosaic virus (TMV)

Fungi

• Can be single-celled (yeasts) or multicellular
• Reproduce by producing spores
• Examples: Candida albicans (thrush), athlete's foot fungus

Protists

• Single-celled eukaryotic organisms
• Often transmitted by vectors
• Example: Plasmodium (malaria, transmitted by mosquitoes)

How diseases spread

Understanding transmission routes is essential for disease prevention and control measures.

Direct contact transmission

• Physical contact between infected and uninfected individuals
• Includes touching, kissing, sexual contact
• Examples: athlete's foot (touching infected surfaces), gonorrhoea (sexual transmission), HIV (exchange of bodily fluids)

Airborne transmission (droplet infection)

• Pathogens carried in tiny water droplets expelled when coughing, sneezing or talking
• Droplets inhaled by others or land on surfaces
• Examples: influenza, tuberculosis, measles, COVID-19

Waterborne transmission

• Contaminated water supplies
• Particularly problematic in areas with poor sanitation
• Example: cholera (Vibrio cholerae)

Food transmission

• Consuming contaminated food
• Often due to poor hygiene or undercooked meat
• Example: Salmonella from contaminated poultry or eggs

Vector transmission

• Pathogens transferred by another organism (the vector)
• Vector is not harmed by the pathogen
• Example: malaria transmitted when female Anopheles mosquitoes bite humans and inject Plasmodium protists

The body's non-specific defence systems

The immune system has multiple layers of defence. Non-specific defences act against all pathogens without distinguishing between them.

Physical and chemical barriers

• Skin: forms a protective barrier; sebaceous glands produce antimicrobial secretions
• Nose: hairs and mucus trap pathogens; mucus contains enzymes that destroy bacteria
• Trachea and bronchi: ciliated epithelial cells waft mucus (containing trapped pathogens) upward to be swallowed
• Stomach: produces hydrochloric acid (pH ~2) that kills most ingested pathogens

White blood cells—phagocytosis

• Phagocytes (a type of white blood cell) patrol the body in the blood and tissues
• Recognise pathogens as foreign
• Engulf pathogens by extending cytoplasm around them
• Form a vacuole (phagosome) inside the cell
• Digest pathogens using enzymes from lysosomes
• This is a non-specific response—works against any pathogen

The body's specific defence system

Specific immunity targets particular pathogens using recognition of antigens.

Lymphocyte response

• Lymphocytes are specialised white blood cells
• Each lymphocyte produces one specific type of antibody
• When a pathogen enters the body, lymphocytes with complementary antibodies are selected
• These lymphocytes rapidly divide (clonal selection) to produce many identical cells
• Plasma cells secrete large quantities of the specific antibody
• Memory cells remain in the blood for long-term immunity

How antibodies work

• Antibodies are Y-shaped proteins with specific binding sites
• Each antibody binds to one specific antigen (lock-and-key mechanism)
• Once bound, antibodies can:
  • Cause pathogens to clump together (agglutination) for easier phagocytosis
  • Neutralise toxins produced by bacteria
  • Mark pathogens for destruction by phagocytes

Primary vs secondary immune response

• Primary response: first exposure to a pathogen—takes several days to produce enough antibodies; symptoms may occur
• Secondary response: subsequent exposure to the same pathogen—memory cells rapidly produce antibodies; faster and stronger response; usually prevents symptoms

This is why you typically only get some diseases (like measles or chickenpox) once.

Vaccination and immunisation

Vaccination provides artificial immunity without causing disease.

How vaccines work

• Contain dead, weakened or fragments of pathogens
• Safe—cannot cause the disease but still carry the antigens
• Immune system responds as if real infection occurred:
  • Lymphocytes recognise antigens
  • Produce specific antibodies
  • Create memory cells
• If real pathogen enters later, memory cells trigger rapid secondary response
• Individual is immune—protected from that disease

Benefits of vaccination programmes

• Prevents individual illness and death
• Reduces disease transmission in populations
• Herd immunity: if enough people are vaccinated, disease cannot spread easily, protecting those who cannot be vaccinated (babies, immunocompromised individuals)
• Can eradicate diseases (smallpox was eliminated globally through vaccination)
• Examples in UK: MMR vaccine (measles, mumps, rubella), HPV vaccine (prevents cervical cancer)

Limitations and considerations

• Does not provide immediate protection—takes time to develop immunity
• Some individuals cannot be vaccinated (allergies, weakened immune systems)
• Immunity may wane over time, requiring booster doses
• Vaccines are pathogen-specific—new vaccines needed for different diseases

Treating and preventing disease

Antibiotics

• Medicines that kill bacteria or prevent their growth
• Work by:
  • Damaging bacterial cell walls
  • Interfering with bacterial protein synthesis or DNA replication
• Do not work against viruses (viruses lack these structures/processes)
• Specific antibiotics effective against specific bacteria
• Must complete full course to ensure all bacteria are killed

Antibiotic resistance

• Major global health concern
• Bacteria can develop resistance through mutations
• Resistant bacteria survive antibiotic treatment and reproduce
• Overuse and misuse of antibiotics increases selection pressure
• Examples: MRSA (Methicillin-resistant Staphylococcus aureus)

Preventing antibiotic resistance

• Only use antibiotics when necessary (not for viral infections)
• Complete full prescribed course
• Avoid unnecessary use in agriculture
• Develop new antibiotics (ongoing research challenge)

Other prevention methods

• Hygiene: handwashing with soap, food preparation safety, sterilising equipment
• Sanitation: clean water supplies, sewage treatment
• Isolation: quarantining infected individuals
• Vector control: using insecticides, mosquito nets (malaria prevention), draining standing water
• Contraception: barrier methods (condoms) prevent STI transmission

Antiviral drugs

• Limited options compared to antibiotics
• Target specific stages of viral replication
• Often slow viral reproduction rather than kill viruses
• Example: antiretrovirals for HIV treatment

Painkillers and other medicines

• Treat symptoms but do not kill pathogens
• Reduce suffering while immune system fights infection
• Examples: paracetamol (reduces fever/pain), cough medicine

Worked examples

Question 1 (3 marks) Describe how phagocytosis protects the body against pathogens.

Mark scheme answer:

• Phagocytes detect/recognise pathogens as foreign (1 mark)
• Phagocyte engulfs the pathogen by extending cytoplasm around it (1 mark)
• Enzymes digest/break down the pathogen inside the cell (1 mark)

Examiner note: Use precise scientific terminology. "Eats" is insufficient—use "engulfs" or "ingests."

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Question 2 (4 marks) Explain why antibiotics can be used to treat bacterial infections but not viral infections.

Mark scheme answer:

• Antibiotics target structures/processes found in bacterial cells (1 mark)
• Such as cell walls or bacterial ribosomes/enzymes (1 mark)
• Viruses lack these structures/do not have their own metabolic processes (1 mark)
• Viruses reproduce inside host cells, making them difficult to target without harming host (1 mark)

Examiner note: "Explain" requires reasons/mechanisms, not just description.

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Question 3 (6 marks) A student received a vaccination against measles. Several years later, they were exposed to the measles virus but did not become ill. Explain how the vaccination protected them.

Mark scheme answer:

• Vaccine contained dead/weakened measles virus or measles antigens (1 mark)
• This triggered the immune system/lymphocytes to respond (1 mark)
• Lymphocytes produced antibodies specific to measles antigens (1 mark)
• Memory cells were produced and remained in the blood (1 mark)
• On second exposure, memory cells rapidly produced antibodies/mounted secondary immune response (1 mark)
• Antibodies destroyed measles virus before symptoms developed (1 mark)

Examiner note: Extended answers require logical sequence. Link vaccination → memory cells → rapid secondary response.

Common mistakes and how to avoid them

• Confusing antibodies and antibiotics: Antibodies are proteins made by white blood cells; antibiotics are medicines that kill bacteria. These are completely different things—learn the distinct definitions.

• Saying vaccines contain the disease: Vaccines contain dead/weakened pathogens or pathogen fragments, not the active disease. They trigger immunity without causing illness.

• Claiming antibiotics kill viruses: This is incorrect and frequently penalised. Antibiotics only work against bacteria because they target bacterial structures that viruses don't have.

• Not distinguishing between non-specific and specific immunity: Phagocytosis is non-specific (works against any pathogen); lymphocyte/antibody response is specific (targets one particular pathogen).

• Forgetting that vectors aren't affected by the pathogen: Mosquitoes carrying malaria parasites don't have malaria—they're just carriers. Vectors transmit but aren't infected.

• Vague descriptions of phagocytosis: Don't write "white blood cells eat bacteria." Use correct terminology: engulf, digest with enzymes, form vacuole.

Exam technique for "Microorganisms and Disease"

• Command word awareness: "Describe" = say what happens (process/observation); "Explain" = say why/how it happens (mechanisms/reasons). "Explain" questions require more depth and usually carry more marks.

• Use biological terminology accurately: Examiner-proof your answers by using specification vocabulary—pathogen (not "germ"), phagocytosis (not "eating"), antibodies, antigens, lymphocytes. Each correct term can secure a mark.

• Extended response structure: For 6-mark questions, write in logical sequence. For immune response: pathogen enters → antigens recognised → lymphocytes selected → antibodies produced → pathogen destroyed → memory cells created.

• Don't over-answer: Each marking point typically = 1 mark. If a question is worth 3 marks, make 3 distinct points. More isn't always better if points aren't relevant or are repetitive.

Quick revision summary

Pathogens (bacteria, viruses, fungi, protists) cause communicable diseases transmitted through contact, air, water, food or vectors. The body defends itself through non-specific mechanisms (skin, stomach acid, phagocytosis) and specific immune responses (lymphocytes producing antibodies against specific antigens). Vaccinations create memory cells for rapid secondary immune responses. Antibiotics kill bacteria but not viruses. Antibiotic resistance develops through natural selection. Prevention methods include hygiene, sanitation, vaccination and vector control.