Infection and response

Welcome to Infection and Response!

Ever wondered why you get a runny nose when you have a cold, or why you need to finish your entire course of antibiotics? In this chapter, we explore the invisible world of microorganisms. We will look at how they try to take over our bodies and the amazing ways our bodies—and modern medicine—fight back. Don’t worry if some of the long names seem tricky at first; we will break them down step-by-step!

4.3.1 Communicable Diseases

What is a Pathogen?

A pathogen is simply a microorganism that causes infectious (communicable) disease. You can think of them as microscopic "unwanted guests" that move from one person to another. Pathogens can be viruses, bacteria, protists, or fungi. They can infect both plants and animals.

How do they spread?
1. By Air: Like droplets from a sneeze (e.g., the flu).
2. By Water: Like drinking contaminated water (e.g., cholera).
3. By Direct Contact: Like touching a surface or another person (e.g., athlete’s foot).

How Bacteria and Viruses Make Us Ill

Bacteria are very small living cells. Once inside you, they reproduce rapidly. They make you feel ill by producing toxins (poisons) that damage your cells and tissues.
Viruses are NOT cells; they are much smaller. They live and reproduce inside your cells, causing the cell to eventually burst and die. This cell damage is what makes you feel sick.

Quick Review Box:

• Bacteria = Produce toxins.
• Viruses = Damage cells from the inside.

Examples of Diseases You Need to Know

Viral Diseases:
• Measles: Symptoms include fever and a red skin rash. It is spread by breathing in droplets from sneezes/coughs. Most children are vaccinated against it because it can be fatal.
• HIV: Initially causes a flu-like illness. If not controlled with drugs, it attacks the immune system. Late-stage HIV is called AIDS, where the body can no longer fight off other infections.
• Tobacco Mosaic Virus (TMV): A plant virus. It creates a "mosaic" pattern of discolouration on leaves. This prevents the plant from carrying out photosynthesis properly, so it can't grow.

Bacterial Diseases:
• Salmonella: Spread by bacteria in food (like undercooked chicken). It causes fever, stomach cramps, vomiting, and diarrhoea.
• Gonorrhoea: A sexually transmitted disease (STD). Symptoms include a thick yellow or green discharge and pain when urinating. It used to be easily treated with penicillin, but many resistant strains have now appeared.

Fungal and Protist Diseases:
• Rose Black Spot (Fungal): Causes purple/black spots on plant leaves. Leaves turn yellow and drop early, meaning less photosynthesis and less growth. It's spread by wind or water.
• Malaria (Protist): Caused by a protist pathogen. It has a complex life cycle involving the mosquito (which acts as a vector—something that carries the disease without getting sick itself). It causes repeating bouts of fever and can be fatal.

Key Takeaway: Pathogens are diverse! They use different methods to travel and different ways to make us sick, but they all rely on a "host" to survive.

4.3.1.6 Human Defence Systems

Your body is like a fortress. It has two main ways to defend itself: keeping pathogens out and destroying them if they get in.

The First Line of Defence (Non-Specific)

These parts of your body try to stop pathogens from entering in the first place:
• Skin: Acts as a physical barrier. If you get a cut, it scabs over to "seal" the breach.
• Nose: Contains hairs and mucus to trap particles containing pathogens.
• Trachea and Bronchi: These tubes secrete mucus to trap pathogens and have tiny hairs called cilia that waft the mucus up to your throat to be swallowed.
• Stomach: Produces hydrochloric acid which is strong enough to kill most pathogens that enter through food or mucus.

The Second Line of Defence (The Immune System)

If a pathogen gets past your skin and stomach acid, your white blood cells take over. They defend you in three ways:
1. Phagocytosis: The cell surrounds, "eats," and digests the pathogen. (Think of it like Pac-Man!)
2. Antibody Production: Every pathogen has unique molecules on its surface. White blood cells produce antibodies (special proteins) that lock onto these molecules and signal for the pathogen to be destroyed. Antibodies are specific—an antibody for the flu won't work on measles.
3. Antitoxin Production: These cancel out the toxins produced by bacteria.

Key Takeaway: Your body uses physical barriers first, then uses specialized white blood cells to hunt and neutralize anything that breaks through.

4.3.1.7 Vaccination

Vaccination is like a "practice run" for your immune system. It involves introducing small amounts of dead or inactive forms of a pathogen into the body.

How it works:
1. The vaccine stimulates white blood cells to produce antibodies.
2. If the same live pathogen enters the body later, the white blood cells "remember" it.
3. They respond quickly to produce the correct antibodies in large numbers, preventing you from getting sick.

Did you know? If a large enough portion of the population is vaccinated, the disease cannot spread easily. This is called herd immunity.

Key Takeaway: Vaccines train your immune system to recognize enemies without you having to actually get the dangerous disease first.

4.3.1.8 Antibiotics and Painkillers

It is very important to know the difference between these two types of medicine!

Antibiotics:
• Medicines (like penicillin) that kill infective bacteria inside the body.
• They have greatly reduced deaths from infectious diseases.
• Crucial point: Antibiotics CANNOT kill viruses. Because viruses live inside your cells, it is very difficult to develop drugs that kill the virus without also damaging the body's tissues.

Painkillers:
• These treat the symptoms of the disease (like a headache or fever) but they do not kill the pathogens. They just make you feel more comfortable while your immune system does the work.

Antibiotic Resistance

Some bacteria have evolved so that antibiotics no longer work on them. This is a huge problem in medicine. To stop this from getting worse, doctors should not prescribe antibiotics for "viral" infections (like a cold), and patients must always finish their whole course of medicine.

Key Takeaway: Antibiotics = Bacteria killers. Painkillers = Symptom hiders. Use them correctly to prevent resistant "superbugs."

4.3.1.9 Discovery and Development of Drugs

In the past, most drugs were extracted from plants or microorganisms:
• Digitalis (heart drug) comes from Foxgloves.
• Aspirin (painkiller) comes from Willow trees.
• Penicillin was discovered by Alexander Fleming from a type of mould.

Today, most new drugs are made by chemists in labs, but the starting point is often still a chemical found in a plant.

Testing New Drugs

New drugs must be tested to make sure they are safe and effective. They are checked for toxicity (is it poisonous?), efficacy (does it work?), and dose (how much is needed?).

The Stages of Testing:
1. Preclinical Testing: Done in a lab using cells, tissues, and live animals.
2. Clinical Trials: Use healthy volunteers and then patients.
• First, very low doses are given to check for safety.
• If safe, further trials find the optimum dose.
• Double-blind trials: Some patients are given the real drug, and some are given a placebo (a "fake" drug that does nothing). Neither the doctor nor the patient knows who has which until the end. This stops bias!

Key Takeaway: Drug testing is a long, careful process to ensure that the medicine helps more than it harms. Remember the order: Cells $\rightarrow$ Animals $\rightarrow$ Humans.

Final Quick Review:

• Pathogens make us ill via toxins or cell damage.
• White blood cells produce antibodies and antitoxins.
• Vaccines use dead pathogens to trigger immunity.
• Antibiotics only work on bacteria.
• New drugs undergo rigorous testing in preclinical and human clinical trials.