Microbes and Disease: KS3 Biology Guide

Microbes are microscopic organisms found almost everywhere on Earth. Some are harmless or even beneficial, but those that cause disease are called pathogens. Understanding how pathogens spread and how the body defends itself is core KS3 biology.

What are microbes?

Microbes (or micro-organisms) are living things too small to be seen with the naked eye. They include:

Bacteria — single-celled prokaryotic organisms (no nucleus). Most are harmless; some cause disease.

Viruses — not true cells; they are strands of genetic material surrounded by a protein coat. All viruses reproduce inside living cells.

Fungi — mostly multicellular; some cause infections in humans (e.g. athlete's foot, thrush).

Protists — single-celled eukaryotes; some are pathogens (e.g. the protist Plasmodium causes malaria).

Only microbes that cause disease are called pathogens.

Bacteria as pathogens

Bacteria cause disease by:

Reproducing rapidly inside the body, damaging tissues.

Producing toxins (poisonous chemicals) that harm cells.

Examples of bacterial diseases

Disease | Bacterium | Transmission
Tuberculosis (TB) | Mycobacterium tuberculosis | Air droplets (coughing, sneezing)
Salmonella food poisoning | Salmonella spp. | Contaminated food (especially poultry and eggs)
Cholera | Vibrio cholerae | Contaminated drinking water
Tetanus | Clostridium tetani | Soil entering wounds

Key distinction: Bacterial diseases can often be treated with antibiotics, which kill bacteria without (at normal doses) harming human cells.

Viruses as pathogens

Viruses cause disease by:

Entering host cells and taking over their machinery.

Replicating — making thousands of copies of themselves inside the cell.

Bursting out of the cell (lysis), destroying it, and infecting new cells.

Examples of viral diseases

Disease | Virus | Transmission
Influenza (flu) | Influenza virus | Air droplets
COVID-19 | SARS-CoV-2 | Air droplets and surfaces
HIV/AIDS | Human immunodeficiency virus (HIV) | Bodily fluids
Measles | Measles morbillivirus | Air droplets

Key distinction: Antibiotics do not work against viruses. Viral infections may be treated with antiviral drugs in some cases, but many are managed by the immune system alone. Vaccination is the most effective preventive strategy.

Fungi as pathogens

Fungal pathogens typically infect the skin, nails, and mucous membranes rather than internal organs in otherwise healthy people. Examples:

Athlete's foot (Tinea pedis) — a fungal infection of the skin between the toes, spread by contact with contaminated surfaces.

Thrush (Candida albicans) — a yeast infection of mucous membranes.

Rose black spot — a fungal disease of rose plants (Diplocarpon rosae), studied as a plant disease example at KS3.

How pathogens spread

Understanding transmission routes allows diseases to be prevented:

Route | Description | Example diseases
Air droplets | Tiny liquid droplets released when coughing, sneezing, or talking | Flu, TB, measles, COVID-19
Contaminated water | Pathogens in drinking or washing water | Cholera, typhoid
Contaminated food | Pathogens in undercooked or poorly stored food | Salmonella, E. coli
Direct contact | Touching an infected person or surface | Athlete's foot, chicken pox
Body fluids | Blood, sexual contact | HIV
Animal vectors | Insects or animals that carry the pathogen | Malaria (mosquito vector)

The body's defences

The human body has a layered defence system:

Barriers (first line of defence)

Skin — a physical barrier that prevents most pathogens from entering. Also produces antimicrobial secretions.

Mucus and cilia — mucus in the respiratory tract traps pathogens; cilia sweep the mucus (and trapped microbes) upward to be swallowed or expelled.

Stomach acid — destroys many pathogens that are swallowed.

The immune system (second line of defence)

If a pathogen enters the body, white blood cells (part of the blood) respond:

Phagocytes engulf and digest pathogens (a process called phagocytosis).

Lymphocytes produce antibodies — proteins that bind specifically to the surface of a pathogen (its antigens). Antibodies neutralise the pathogen or flag it for destruction.

After infection, some lymphocytes become memory cells that remain in the blood. If the same pathogen enters the body again, the immune response is much faster — this is immunity.

Worked example: what happens when influenza virus enters the body

Influenza viruses enter airway cells and replicate rapidly (symptoms: fever, aches — caused partly by the immune response itself).

Phagocytes engulf virus particles and present antigens.

Lymphocytes recognise the antigen and produce specific antibodies.

Antibodies bind to the virus and prevent it infecting more cells.

Recovery occurs after 7–10 days. Memory cells remain.

Next influenza exposure of the same strain: memory cells react within hours — the person may not develop symptoms at all.

Vaccination

A vaccine introduces a harmless form of the pathogen's antigen (weakened or dead pathogen, or just its protein coat) into the body.

The immune system responds normally: lymphocytes produce antibodies and memory cells form.

No serious illness occurs because the pathogen cannot replicate properly.

If the real pathogen later infects the person, memory cells produce a rapid, large antibody response before symptoms develop.

Herd immunity occurs when a large enough proportion of the population is vaccinated, protecting unvaccinated individuals because the pathogen cannot spread easily.

According to the Department for Education's KS3 Science Programme of Study, pupils should learn about the relationship between health, disease, and the human body's defence systems, including the role of vaccination, antibiotics, and the immune response. BBC Bitesize KS3 Biology covers pathogens, the immune system, and vaccination as core Year 8 content within its microorganisms topic.

Frequently asked questions

What is the difference between a bacterium and a virus?

Bacteria are living single-celled organisms with their own metabolism — they can reproduce independently and produce toxins. Viruses are non-living outside a host cell; they are genetic material (DNA or RNA) in a protein coat and can only replicate by taking over a host cell's machinery. This is why antibiotics can kill bacteria (by disrupting bacterial cell processes) but have no effect on viruses.

Why do antibiotics not work on viruses?

Antibiotics work by targeting structures or processes unique to bacteria — for example, attacking the bacterial cell wall, which human cells do not have. Viruses do not have cell walls or independent metabolism, so antibiotics have nothing to act on. This is why it is important not to take antibiotics for viral illnesses such as colds and flu — it does not help and contributes to antibiotic resistance.

What does the immune system remember after an infection?

After an infection, a small number of lymphocytes that produced antibodies remain in the blood as memory cells for many years (sometimes for life). They carry information about the specific antigens on the pathogen. If the same pathogen enters the body again, memory cells multiply rapidly and produce antibodies much faster than the first time, often stopping symptoms before they develop.

How does vaccination protect people who are not vaccinated?

When enough people in a community are immune (either through vaccination or previous infection), a pathogen struggles to find new hosts and cannot spread. This herd immunity means even people who cannot be vaccinated — such as newborn babies or those with certain medical conditions — are protected because the pathogen is unlikely to reach them.

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