Welcome to Chapter B2.1: What are the Causes of Disease?

In this chapter, we are going to dive into why we get sick and what causes those "off days" where we don't feel our best. Understanding the causes of disease is the first step in learning how to stay healthy. This is a core part of your OCR Twenty First Century Science Biology course, and it’s actually very relevant to your everyday life!

Don't worry if this seems like a lot of information at first. We will break it down into small, bite-sized pieces and use some simple tricks to help you remember the key facts.


1. Health vs. Disease: What’s the Difference?

To understand disease, we first need to know what health is. Think of your body like a high-performance car. When everything is working perfectly, the car is "healthy." If a part breaks or the fuel is contaminated, the car's performance drops—that’s like having a disease.

Key Definition: Health is a state of physical and mental well-being. A disease is a condition that compromises this health, damaging cells and stopping parts of the body from working as they should.

The Incubation Period (The "Stealth Mode")

Have you ever felt fine one day and then woken up the next day feeling terrible? That’s because of the incubation period. This is the time between when a pathogen (a germ) enters your body and when you actually start showing symptoms. During this time, the pathogen is busy reproducing inside you, but hasn't caused enough damage yet for you to feel it.

Key Takeaway: Disease damages cells and impairs functions, leading to symptoms. However, you might be infected and not even know it yet!


2. The Two Main Types of Disease

Not all diseases are the same. We split them into two big groups:

A. Communicable Diseases
These are infectious diseases caused by pathogens (microorganisms like bacteria or viruses). They can be passed from one organism to another. Example: The flu or a cold.

B. Non-Communicable Diseases
These cannot be passed from person to person. They are usually caused by a combination of your genes, your lifestyle (like diet or exercise), or your environment. Example: Heart disease or asthma.

Quick Review: If you can "catch it" from a friend, it’s communicable. If it’s caused by your genes or lifestyle, it’s non-communicable.


3. Meet the Pathogens: The "Bad Guys"

A pathogen is just a fancy scientific word for a microorganism that causes disease. There are four main types you need to know:

1. Bacteria: Tiny single-celled organisms. Not all are bad, but the pathogenic ones release toxins that damage your cells.
2. Viruses: Even smaller than bacteria. They "hijack" your cells, live inside them, and reproduce until the cell bursts.
3. Protists: Single-celled eukaryotic organisms (they have a nucleus). Many are parasites.
4. Fungi: These can be single-celled or made of thread-like structures called hyphae.

Memory Aid: Use the mnemonic B.V.P.F. — Bad Villains Produce Fear (Bacteria, Viruses, Protists, Fungi).


4. Common Human Infections

The syllabus requires you to know these specific examples. Let's look at who causes them:

Influenza (Flu): Caused by a virus. It spreads through the air when people cough or sneeze.
Salmonella: Caused by bacteria. You usually get this from eating "poisoned" food that hasn't been cooked properly.
Athlete's Foot: Caused by a fungus. It makes the skin between your toes itchy and flaky.
Malaria: Caused by a protist. It is spread by mosquitoes (which we call vectors).
HIV/AIDS: Caused by a virus. This is a Sexually Transmitted Infection (STI) that attacks the immune system.

Did you know? A vector is like a delivery driver for a disease. The mosquito doesn't get sick from Malaria, but it delivers the protist to humans!


5. Diseases in Plants

It’s not just humans that get sick—plants do too! This is very important for farmers who need to grow our food. You need to know these three:

1. Tobacco Mosaic Virus (TMV): Caused by a virus. It creates a "mosaic" pattern of discolouration on leaves, which stops the plant from photosynthesising properly.
2. Ash Dieback: Caused by a fungus. It leads to leaf loss and bark lesions in ash trees.
3. Crown Gall Disease: Caused by bacteria. It causes large, tumor-like growths (galls) on the roots or stems of plants.

Key Takeaway: Plant diseases often affect the leaves or stems, which can stop the plant from growing and eventually kill it.


6. How are Diseases Spread?

Pathogens are very good at traveling. Here are the main routes they take:

  • Bodily Fluids: Such as blood or saliva (e.g., HIV or Colds).
  • Surfaces: Touching a doorknob that an infected person touched (e.g., Athlete's foot or the flu).
  • Food and Water: Drinking dirty water or eating undercooked chicken (e.g., Salmonella).
  • Airborne: Breathing in tiny droplets from a sneeze (e.g., Influenza).

Common Mistake to Avoid: Don't confuse the cause with the spread. The cause is the pathogen (e.g., bacteria), but the spread is how it gets to you (e.g., in water).


Chapter Summary: Quick Review

- Health is total well-being; disease breaks that health.
- Pathogens (Bacteria, Viruses, Protists, Fungi) cause communicable diseases.
- Incubation Period is the time between infection and symptoms.
- Human examples: Flu (virus), Salmonella (bacteria), Athlete's foot (fungi), Malaria (protist), HIV (virus).
- Plant examples: TMV (virus), Ash dieback (fungi), Crown gall (bacteria).
- Spread happens via air, water, contact, or vectors.