Germ Warfare

Germ Warfare ⚔️

Every day your body is under attack from **pathogens** — and it fights back. This module maps the whole battle: the invaders, the diseases they cause, your defences, and the weapons (vaccines and antibiotics) that turn the tide. Big topic — but the marks come from **naming the specific pathogen and example** each time.

Know your enemy 🦠

A **pathogen** is a microorganism that causes **communicable** (infectious) disease. There are four types: **viruses, bacteria, fungi and protists**. They spread by **direct contact**, in **water**, in **food**, through the **air** (droplets), or via a **vector** (a carrier such as an insect).

What is a pathogen?

Which is the best definition of a **pathogen**?

• A microorganism that causes communicable disease
• Any microorganism
• A white blood cell that fights infection
• A protein that fights infection

Type and example

Viral diseases 🧬

Three viruses to know: • **Measles** — spread by inhaling **droplets** from coughs and sneezes; fever and a red skin rash; can be dangerous. Prevented by vaccination (MMR). • **HIV** — spread by **sexual contact** or exchange of body fluids (e.g. shared needles). It attacks immune cells; untreated it leads to **AIDS**. Controlled with antiretroviral drugs. • **Tobacco mosaic virus (TMV)** — a **plant** virus giving leaves a mosaic discolouration that reduces photosynthesis and growth.

How does it spread?

How is **measles** spread from person to person?

• By inhaling droplets from coughs and sneezes
• By sexual contact
• By eating contaminated food
• By a mosquito bite

Bacteria, fungi and protists 🍗

The rest of the rogues' gallery: • **Salmonella** (bacterium) — food poisoning from contaminated food; fever, cramps, vomiting and diarrhoea caused by **toxins**. UK poultry are vaccinated against it. • **Gonorrhoea** (bacterium) — a sexually transmitted disease; treated with **antibiotics** and prevented with condoms. • **Rose black spot** (fungus) — black spots on rose leaves that then yellow and drop; spread in water and wind; treated with fungicides. • **Malaria** (protist) — spread by a **mosquito vector**; recurrent fevers. Prevented by stopping mosquitoes breeding and using nets.

How each spreads

Your first line of defence 🛡️

Before the immune system even engages, the body has **non-specific barriers**: • **Skin** — a physical barrier that also makes antimicrobial secretions. • **Nose** — hairs and mucus trap particles. • **Trachea & bronchi** — mucus traps pathogens; **cilia** waft it away from the lungs. • **Stomach** — hydrochloric **acid** kills swallowed pathogens.

How each defends

The immune system 🩸

If pathogens get past the barriers, **white blood cells** attack in three ways: • **Phagocytosis** — a white blood cell engulfs and digests the pathogen. • **Antibodies** — specific proteins that lock onto the pathogen's antigens, clumping them together (agglutination) and marking them for destruction. • **Antitoxins** — neutralise the toxins that pathogens release.

What do antibodies do?

Exactly how do **antibodies** help defeat a pathogen?

• They lock onto it, clump them together and mark them for destruction
• They directly poison and kill the pathogen
• They engulf and digest the pathogen
• They neutralise the pathogen's toxins

Vaccination and herd immunity 💉

A **vaccine** contains dead or inactive pathogen (its antigens). Your white blood cells make **antibodies** against it, and **memory cells** stay behind. If the real pathogen invades later, the memory cells make antibodies **faster and in greater quantity** — the **secondary response** — destroying it before you get symptoms. If enough of the population is vaccinated, the disease can't spread easily — **herd immunity** protects even the unvaccinated.

Order the response

An interactive activity.

Antibiotics vs painkillers 💊

Two very different medicines: • **Antibiotics** (e.g. penicillin) **kill bacteria** inside the body — but do **nothing to viruses**. • **Painkillers** relieve **symptoms** but don't touch the pathogen. Overusing antibiotics breeds **resistant bacteria** (like MRSA): by **natural selection**, the few resistant bacteria survive and reproduce while the rest die, so the resistant strain spreads.

True of antibiotics?

Pick the TWO correct statements about antibiotics.

• They kill bacteria but have no effect on viruses
• Overusing them leads to resistant strains by natural selection
• They relieve pain without killing the pathogen
• They can cure viral diseases like measles

Testing new drugs 🎓

New drugs are trialled in stages before use: **preclinical** testing on **cells, tissues and animals**, then **clinical trials** on **healthy volunteers** (low doses, checking safety) and then **patients** (finding the optimum dose). Trials use a **placebo** and are **double-blind** — neither doctor nor patient knows who got the real drug. **Exam recap:** always name the **pathogen type AND example**; antibodies **mark/agglutinate** (they don't kill directly); the **secondary response** is faster and bigger; and antibiotic **resistance evolves by natural selection**.