The causes of disease: pathogens, lifestyle and genes

Welcome to Your Guide on the Causes of Disease!

In this chapter, we are going to explore why we get sick. It isn’t always just down to "germs." We will look at how pathogens (like bacteria and viruses) attack us, how our own lifestyle choices (like diet and smoking) affect our hearts, and how changes in our genes can lead to cancer. Understanding these causes helps doctors find better ways to treat and prevent illness.

Don't worry if some of the scientific terms look a bit intimidating at first. We will break them down step-by-step with simple analogies and clear explanations!

1. Pathogens: The Biological Invaders

A pathogen is simply a biological agent that causes disease. The two main types you need to know for this course are bacteria and viruses.

How Pathogens Enter the Body

Our bodies have "interfaces" with the environment—places where the inside of our body meets the outside world. Pathogens try to sneak through these gaps. In mammals, the most common entry points are:

• The Digestive System: Through contaminated food or water.
• The Gas-Exchange System: Through droplets in the air we breathe.
• The Reproductive System: Through physical contact.

How They Make Us Sick

Once inside, pathogens cause damage in two main ways:

1. Damaging Host Cells: They can physically break into cells or use up the cell's resources until the cell dies. For example, viruses hijack your cells to make copies of themselves.

2. Producing Toxins: Many bacteria release chemical poisons (toxins) that interfere with how your body functions.

Quick Review: Pathogens = Bacteria or Viruses. They get in through interfaces and cause harm by cell damage or toxins.

2. Lifestyle and Coronary Heart Disease (CHD)

Sometimes, disease isn't caused by an invader, but by how our body changes over time due to our lifestyle. Coronary Heart Disease (CHD) is a major example of this.

The Biological Basis: Atheroma

The root cause of most heart disease is an atheroma. Think of an atheroma like a "fatty clog" inside the wall of an artery. It is made of lipid-containing material.

The Danger of Atheroma:

• Aneurysm: The atheroma weakens the artery wall, causing it to bulge like a weak spot on a balloon. This bulge can burst, leading to internal bleeding.
• Thrombosis: If the atheroma breaks through the artery lining, it can trigger a blood clot (thrombosis). This clot can completely block the blood flow.
• Myocardial Infarction (Heart Attack): If the blood flow to the heart muscle itself is interrupted (usually by a clot), the muscle cells die because they don't get oxygen. This is a heart attack.

Risk Factors: Why Does It Happen?

A risk factor is something that increases your chance of getting a disease. For CHD, these include:

• Diet: High levels of saturated fats can increase blood cholesterol.
• Blood Cholesterol: High "bad" cholesterol leads to more atheromas.
• Cigarette Smoking: Chemicals in smoke damage artery linings and make blood stickier.
• High Blood Pressure: Puts extra stress on the artery walls, making them more likely to develop atheromas.

Did you know? Lifestyle changes, like exercising more and eating less saturated fat, can significantly reduce your risk of CHD even if you have a family history of it!

Key Takeaway: CHD starts with a fatty atheroma in the artery wall, which can lead to clots (thrombosis) and eventually a heart attack (myocardial infarction).

3. Genes, Mutation, and Cancer

Sometimes, the "instructions" inside our cells (our DNA) get corrupted. This is called a mutation.

What is a Gene Mutation?

A mutation is a change in the base sequence of DNA. They happen spontaneously during DNA replication (when cells copy themselves). However, mutagens (like UV light, X-rays, or certain chemicals) can make these mistakes happen more often.

Two Common Types of Mutation:

1. Base Substitution: One "letter" in the DNA code is swapped for another. Example: Changing "CAT" to "CAR."

2. Base Deletion: One "letter" is completely removed. This is often more serious because it shifts the whole code afterward.

How Mutations Lead to Cancer

Cancer is essentially uncontrolled cell division. Your body has two types of genes that act like the "pedals" of a car to control how fast cells divide:

• Proto-oncogenes: These are like the gas pedal. They tell the cell to divide. If they mutate into oncogenes, the "pedal" gets stuck down, and the cell divides too fast.
• Tumour Suppressor Genes: These are like the brakes. They slow down cell division. If these are mutated and "inactivated," the cell loses its ability to stop dividing.

Benign vs. Malignant Tumours

When cells divide out of control, they form a mass called a tumour.

• Benign Tumours: These grow slowly and stay in one place. They are usually not life-threatening but can put pressure on organs.
• Malignant Tumours: These are cancerous. They grow rapidly, invade nearby tissues, and can spread to other parts of the body through the blood.

Memory Aid: Think of Benign as "Behaves" (stays put) and Malignant as "Moving" (spreads and causes harm).

Summary Checklist

To master this chapter, make sure you can:

1. Define a pathogen and explain the two ways they cause disease (cell damage and toxins).
2. Describe how an atheroma can lead to an aneurysm, thrombosis, or myocardial infarction.
3. Identify lifestyle risk factors for heart disease and explain how they impact the body.
4. Explain the difference between a base substitution and a base deletion mutation.
5. Describe the roles of oncogenes and tumour suppressor genes in the development of cancer.

Don't worry if you need to read through this a few times! Biology is all about connections, and once you see how a "clogged pipe" (atheroma) leads to a "broken pump" (heart attack), it all starts to make sense.