Introduction to Communicable Diseases

Pathogens are basically the troublemakers of the microscopic world - they're microorganisms that cause infectious diseases in both plants and animals. There are four main types you need to know: viruses, bacteria, protists, and fungi.

Viruses are tiny invaders that hijack your cells like biological pirates. They sneak into your cells, use the cell's own machinery to make copies of themselves, then burst the cell open to release hundreds of new viruses into your bloodstream. This cellular destruction is what makes you feel rough.

Bacteria are slightly larger and multiply incredibly quickly through binary fission (basically splitting in two). They damage your body by producing toxins - poisonous substances that harm your cells and make you feel ill.

💡 Quick Tip: Remember the three main ways diseases spread - direct contact (like kissing), through water, and through air (like when someone sneezes on you). Gross but important!

Viral Diseases You Need to Know

Viruses are particularly sneaky because they can invade any type of cell, and we still haven't developed medicines that can cure them. Your body has to fight them off naturally.

Measles spreads through droplet infection (when infected people cough or sneeze) and causes fever plus that characteristic red rash. It can lead to serious complications like pneumonia or brain infections. The good news? Childhood vaccinations have made it much less common.

HIV is a sexually transmitted virus that initially feels like flu, but then attacks your immune system. Without treatment, it progresses to AIDS, leaving your body vulnerable to other diseases. Antiretroviral drugs can now stop the virus from multiplying, turning HIV from a death sentence into a manageable condition.

Tobacco mosaic virus affects plants, causing yellow and green patches on leaves. This discolouration stops affected areas from photosynthesising properly, reducing crop yields and costing farmers money.

💡 Remember: Unlike bacterial infections, viral diseases can't be cured with antibiotics - your immune system has to do the hard work!

Bacterial and Other Diseases

Bacterial diseases are becoming increasingly problematic because many bacteria are developing antibiotic resistance. This means medicines that used to work are becoming useless.

Salmonella lives in animal guts and gets into our food through raw meat and eggs. It produces toxins that cause fever, stomach cramps, and that lovely combination of vomiting and diarrhoea. Prevention is key - cook meat thoroughly and keep raw meat away from other foods.

Gonorrhoea is a sexually transmitted bacterial infection causing painful urination and discharge. Condoms prevent transmission, and while antibiotics can treat it, many strains are becoming resistant to penicillin.

Rose black spot is a fungal disease that creates black spots on rose leaves, reducing photosynthesis and causing leaves to drop early. Malaria is caused by protist parasites carried by female mosquitoes - when they bite you, the parasites enter your bloodstream and reproduce inside your red blood cells.

💡 Key Point: Each type of pathogen has different prevention methods - bacteria respond to antibiotics, but fungi need fungicides and parasites need vector control!

Your Body's Defence System

Your body is like a fortress with multiple layers of defence against pathogens. The non-specific defence system tries to stop pathogens getting in, while the specific immune system targets particular invaders.

Your skin acts as a physical barrier and produces antimicrobial substances. Your nose has hairs and mucus to trap particles, while your trachea and bronchi use cilia $tiny hair-like structures$ to sweep mucus upwards so you can swallow it. Your stomach then kills pathogens with hydrochloric acid.

White blood cells are your body's special forces, working in three ways. Phagocytosis involves engulfing and destroying pathogens. They also produce antibodies - specific proteins that bind to antigens on pathogen surfaces, making them clump together for easier destruction.

The brilliant thing about antibodies is immunological memory. Once your white blood cells have made antibodies against a pathogen, they can produce them much faster if you're infected again - that's why you usually only get chickenpox once!

💡 Amazing Fact: Your immune system can remember thousands of different pathogens and respond to them within hours of a repeat infection!

Vaccination and Drug Development

Vaccination is basically giving your immune system a practice run. Vaccines contain dead or weakened pathogens that stimulate antibody production without making you ill. When you encounter the real disease, your immune system is ready to fight it immediately.

Herd immunity occurs when enough people are vaccinated to protect the whole community. Even people who can't be vaccinated are protected because the disease can't spread easily. Vaccines have completely eradicated smallpox and dramatically reduced diseases like rubella.

Many modern drugs originally came from plants and microorganisms. Aspirin comes from willow trees, while penicillin was discovered when Alexander Fleming noticed mould killing bacteria on his culture plates. This accidental discovery revolutionised medicine.

New drugs undergo rigorous testing - first preclinical testing on cells and animals, then clinical trials on human volunteers and patients. Double-blind trials ensure neither patients nor doctors know who's receiving the real drug, eliminating bias in results.

💡 Remember: Antibiotics only work on bacteria, not viruses. Taking antibiotics unnecessarily helps bacteria develop resistance!

Monoclonal Antibodies

Monoclonal antibodies are identical antibodies produced from the same immune cell. Think of them as highly targeted missiles that can find and bind to one specific target in your body.

Scientists create them by combining mouse white blood cells (which make antibodies but can't divide) with tumour cells (which divide rapidly but don't make antibodies). The resulting hybridoma cells can both divide and produce antibodies.

Pregnancy tests use monoclonal antibodies to detect hCG hormone in urine. The test strip has mobile antibodies attached to blue beads that bind to hCG, then get caught by stationary antibodies further along, creating a blue line if you're pregnant.

In cancer treatment, monoclonal antibodies can target tumour markers - antigens found only on cancer cells. They can stimulate your immune system to attack cancer cells, block growth signals, or deliver toxic drugs directly to tumours while leaving healthy cells alone.

The main challenge is that monoclonal antibodies originally came from mice, so human immune systems sometimes attacked them. Scientists now create mouse-human hybrid antibodies to solve this problem.

💡 Cool Application: Monoclonal antibodies can be used to screen blood donations for diseases like HIV, making blood transfusions much safer!

Plant Disease and Defence

Plants face their own disease challenges from pathogens, plus problems from nutrient deficiencies and pests. Recognising these issues is crucial for farmers and gardeners.

Common signs of plant disease include stunted growth (often indicating nitrate deficiency), spots on leaves (like black spot fungus), areas of decay, abnormal growths, and discolouration. You can identify specific diseases using gardening manuals, laboratory analysis, or monoclonal antibody tests.

Nitrate deficiency stunts growth because plants need nitrates to convert sugars from photosynthesis into proteins required for growth. Magnesium deficiency causes chlorosis - yellow and green patches on leaves - because magnesium is essential for making chlorophyll.

Plants have evolved impressive defence mechanisms. Physical defences include tough waxy cuticles that prevent pathogen entry and cellulose cell walls that form barriers around cells. These adaptations help plants survive in environments full of potential threats.

💡 Farmer's Tip: Healthy soil with proper nutrients is the best defence against plant disease - prevention is always better than treatment!