Disease Definitions and Key Concepts

Ever wondered why some diseases spread like wildfire whilst others stay contained? It all comes down to understanding how pathogens $disease-causing organisms$ behave and spread.

Infectious diseases can jump from person to person, but here's the tricky bit - some people become carriers who spread disease without showing any symptoms themselves. These invisible spreaders make disease control seriously challenging.

Diseases exist on different scales too. Endemic diseases hang around constantly in certain areas at low levels, like background noise. When cases suddenly spike and spread rapidly, that's an epidemic. Scale that up globally and you've got a pandemic - think COVID-19 or the 1918 flu.

The weapons in our arsenal include vaccines (which train your immune system to recognise threats) and antibiotics (which directly attack bacteria). However, antibiotic resistance is becoming a massive problem as bacteria evolve to survive our best treatments.

Quick Tip: Remember that antibiotics only work on bacteria, not viruses - that's why you can't treat flu with antibiotics!

Major Infectious Diseases

These five diseases show how different pathogens cause havoc in completely different ways. Cholera hits through contaminated water, causing deadly dehydration from severe diarrhoea. Prevention focuses on clean water and good hygiene.

Tuberculosis spreads through airborne droplets and loves to attack your lungs. It's particularly nasty because it's linked to HIV infections. The BCG vaccine helps protect kids, but treatment requires a long slog of antibiotics.

Smallpox was so successfully eradicated that we stopped vaccinating against it. This DNA virus had low antigenic variation, meaning it didn't mutate much, making vaccines incredibly effective.

Influenza is the opposite - it constantly changes its surface proteins, which is why you need yearly flu jabs. There are three main types (A, B, C) with loads of different strains.

Malaria shows how vectors (disease carriers) work. Female mosquitos inject the parasite into your bloodstream, where it wreaks havoc on your liver and red blood cells. Prevention focuses on controlling mosquito populations.

Exam Focus: Know which diseases are caused by bacteria, viruses, or parasites - this comes up frequently in questions!

How Pathogens Cause Disease

Viruses are basically cellular hijackers with two main strategies. The lytic cycle means immediate takeover - they use your cell's machinery to make copies, then burst the cell open to release new viruses. Lysogenic viruses are sneakier, hiding in your DNA for ages before activating.

Viruses cause damage through four key mechanisms. Cell lysis literally makes cells explode from internal pressure. Many viral components are toxic to your cells. Cell transformation can trigger cancer if viral DNA messes with your growth-control genes. Immune suppression (like HIV) destroys your defence system entirely.

Antimicrobials include everything that fights microbes - antiseptics, disinfectants, and antibiotics. But antibiotics specifically target bacteria without harming your own cells.

Broad-spectrum antibiotics like tetracycline work against loads of different bacteria. Narrow-spectrum ones are more targeted. Some are bactericidal (kill bacteria outright) whilst others are bacteriostatic (just stop them reproducing).

The key difference between Gram-positive and Gram-negative bacteria lies in their cell wall structure. This affects which antibiotics work - penicillin smashes Gram-positive walls but struggles with Gram-negative bacteria's protective outer layer.

Memory Trick: Gram-positive = purple stain, thick wall, penicillin works well!

Antibiotic Action and Resistance

Penicillin works by sabotaging bacterial cell wall construction. It blocks the enzymes that create cross-links in peptidoglycan, so when bacteria try to grow, their weakened walls burst under osmotic pressure. It's like removing the steel rebar from concrete.

Tetracycline takes a different approach, targeting protein synthesis. It jams up bacterial ribosomes by blocking where tRNA molecules should attach, effectively stopping bacteria from making essential proteins.

Antibiotic resistance develops through two main routes. Random mutations during DNA replication might create resistance by chance. More worryingly, bacteria can pick up plasmids (extra DNA circles) carrying resistance genes from their environment - it's like sharing cheat codes.

Here's the scary bit: once resistance emerges, resistant bacteria have a huge survival advantage whenever antibiotics are present. Even when antibiotics aren't around, these resistant strains stick around, ready to cause untreatable infections.

Real-World Impact: MRSA $methicillin-resistant Staphylococcus aureus$ is a perfect example of how antibiotic resistance creates "superbugs" that are incredibly difficult to treat.

Your Body's Defence Systems

Your innate immune system is like a castle's outer defences - always there, ready for anything. Your skin forms the main barrier, helped by friendly skin flora that outcompete nasty bacteria. When barriers break, blood clotting seals wounds whilst inflammation localises threats.

Other innate defences include phagocytosis (cells that literally eat invaders), ciliated mucous membranes that trap germs, and lysozyme in tears and saliva that dissolves bacterial walls.

The adaptive immune system is your body's special forces - it learns and remembers specific threats. The humoral response involves B lymphocytes making antibodies - Y-shaped proteins that bind to specific antigens and mark them for destruction.

The cell-mediated response uses T lymphocytes with different jobs. T helper cells coordinate attacks, T killer cells directly destroy infected cells, and T memory cells remember threats for next time.

Your primary immune response is slow - takes 2-3 weeks to get going on first exposure. But the secondary response is lightning fast thanks to memory cells, producing 10-100 times more antibodies almost instantly.

Exam Essential: Primary = slow, low antibodies, symptoms occur. Secondary = fast, high antibodies, no symptoms!

Immunity and Vaccination

Active immunity means your body makes its own antibodies, either naturally (after infection) or artificially (after vaccination). This gives long-lasting protection because you keep memory cells. Passive immunity involves receiving ready-made antibodies from someone else - quick protection but short-lived.

Passive immunity happens naturally (mum's antibodies via placenta or breast milk) or artificially (antibody injections for rabies or tetanus). It's perfect for emergencies when you can't wait for your immune system to gear up.

Successful vaccines need two key features. The antigen must be highly immunogenic (triggers strong immune response) and there should ideally be only one antigenic type of the pathogen. This is why rubella vaccination works brilliantly but influenza vaccines struggle.

Flu is a nightmare because it has multiple antigenic types and constantly mutates. Memory cells from last year's vaccine might not recognise this year's slightly different virus - hence annual flu jabs with educated guesses about which strains will dominate.

Vaccination isn't compulsory in the UK, respecting individual choice. However, this creates ethical dilemmas, especially for healthcare workers who might harm patients by not being vaccinated.

Critical Thinking: Consider both individual rights and public health - vaccination protects not just you but vulnerable people who can't be vaccinated due to medical conditions.