B3 Infection and Response - GCSE AQA Biology Revision Notes

What Are Communicable Diseases?

Pathogens are nasty microorganisms that invade your body and make you ill. They come in four main types, each with their own sneaky ways of causing trouble.

Bacteria multiply incredibly fast inside you and pump out toxins that damage your cells and tissues. Meanwhile, viruses are even more devious - they hijack your cells' machinery to make copies of themselves, then burst the cell open to release new viruses everywhere.

Protists are single-celled parasites that often hitch rides on vectors (like mosquitoes) to reach new hosts. Fungi can be single-celled or made of thread-like structures called hyphae that penetrate skin and plant surfaces, spreading through spores.

Quick Tip: Remember that pathogens spread through dirty water, air droplets from coughs and sneezes, and direct contact with contaminated surfaces.

Common Viral Diseases You Should Know

Measles spreads through droplets when infected people cough or sneeze, causing red rashes and fever. It can be deadly if it leads to pneumonia, but most people get vaccinated as children to prevent it.

HIV spreads through sexual contact or sharing needles, initially causing flu-like symptoms before attacking your immune system. Antiretroviral drugs can control it during the symptom-free years, but late-stage HIV becomes AIDS.

The tobacco mosaic virus affects plants, creating mosaic patterns on leaves that reduce photosynthesis and stunt growth. This shows how diseases don't just affect humans - they impact the entire ecosystem.

Remember: Vaccination is your best defence against viral diseases since antibiotics don't work on viruses.

Fungal, Protist & Bacterial Diseases

Rose black spot is a fungal disease causing purple-black spots on rose leaves, which then yellow and drop off. It spreads through water and wind, but you can treat it with fungicides and removing affected leaves.

Malaria comes from protists carried by mosquitoes. When mosquitoes bite infected animals, they pick up the protist and inject it into their next victim, causing recurring fever episodes. Prevention focuses on mosquito nets and insecticides.

Salmonella bacteria cause food poisoning with fever, cramps, and vomiting from their toxins. Gonorrhoea is a sexually transmitted bacterial disease causing painful urination and discharge - now harder to treat due to antibiotic resistance.

Key Point: Each pathogen type requires different treatment approaches, so identifying the cause is crucial for effective treatment.

Your Body's Defence System

Your body has brilliant built-in defences that work 24/7 to keep pathogens out. Your skin acts like a fortress wall, whilst your nose produces mucus to trap particles and your stomach's hydrochloric acid destroys swallowed pathogens.

Your immune system uses white blood cells as microscopic soldiers. They engulf pathogens through phagocytosis, produce antibodies that lock onto invaders, and create antitoxins to neutralise bacterial poisons.

Vaccination works by introducing dead or inactive pathogens, training your immune system to recognise real threats. It's prevented countless epidemics, though some people worry about rare side effects like swelling or fever.

Prevention beats cure every time - wash your hands regularly, destroy vector breeding sites, isolate infected individuals, and get vaccinated when recommended.

Top Tip: Your immune system has an amazing memory - once it learns to fight a pathogen, it remembers how to defeat it quickly next time.