Biology (9610) | The causes of disease: pathogens, lifestyle and genes

The Causes of Disease: Pathogens, Lifestyle, and Genes

Hello future Biologists! This chapter is incredibly relevant to the real world, as we explore why people get sick. We are moving beyond the cells and molecules of Unit 1 to look at how biological systems are affected by disease.

Understanding the causes of disease isn't just theory; it helps us develop treatments, preventative measures, and make informed choices about our own health. Don't worry if some of these concepts feel familiar from general knowledge—we are going to break down the specific biological mechanisms. Let's get started!

Section 1: Pathogens – The Agents of Infection (3.2.1.1)

Infectious diseases are caused by agents that invade the body. These agents are known as pathogens.

What are Pathogens?

Pathogens are typically microorganisms or acellular entities that cause disease. The syllabus specifically focuses on two main types:

• Bacteria: These are single-celled prokaryotic organisms. They cause disease through rapid multiplication and toxin production. (Example: Cholera, discussed in detail later.)
• Viruses: These are much smaller than bacteria and are acellular (not true cells). They must invade a host cell to replicate. (Example: HIV, discussed in detail later.)

How Pathogens Enter the Body

For a pathogen to cause harm, it must first gain entry by penetrating one of the organism’s interfaces with the environment. Think of the body's surfaces as protective barriers; pathogens look for ways to breach them.

In mammals, the three primary interfaces susceptible to invasion are:

1. Digestive System: Entry via food or water (e.g., cholera bacteria ingested).
2. Gas-Exchange System: Entry via inhalation (e.g., airborne viruses entering the lungs).
3. Reproductive System: Entry via sexual contact (e.g., HIV).

Analogy: Imagine your body is a fortress. The skin is the thick main wall, but the digestive, gas-exchange, and reproductive systems are necessary entry points (gates) that are often easier targets for invaders.

How Pathogens Cause Disease

Once inside, pathogens cause disease in two primary ways:

1. Damaging Host Cells Directly:
Viruses, for example, burst out of cells after replication, destroying the host cell completely. Bacteria can also physically invade and damage tissues.

2. Producing Toxins:
Many bacteria release chemical substances called toxins. These toxins are often highly poisonous and interfere with the host cell's metabolism, causing damage far away from where the bacteria are growing. (Example: Cholera toxins cause severe water imbalance in the intestine.)

Section 2: Lifestyle and Non-Communicable Diseases (3.2.1.2)

Not all diseases are caused by external pathogens. Many major health issues today are non-communicable, meaning they are not passed from person to person. These often arise from a complex interplay of genetic factors, environmental factors, and lifestyle choices.

The Big Two Lifestyle Diseases

The syllabus highlights two key non-communicable diseases associated with specific risk factors:

1. Coronary Heart Disease (CHD)
2. Cancer

Making changes in lifestyle is one of the most effective ways to reduce the risk of contracting these conditions.

Section 3: Coronary Heart Disease (CHD) (3.2.7.1 & 3.2.7.2)

Coronary Heart Disease (CHD) refers to the disease of the coronary arteries, which supply oxygenated blood to the cardiac (heart) muscle itself. If the heart muscle doesn't receive oxygen, a myocardial infarction (heart attack) can occur.

The Biological Basis of CHD

The core problem in CHD is a condition called atheroma.

1. Atheroma Formation: This is the presence of lipid-containing material (including cholesterol) within the walls of the arteries. This material accumulates, forming a hard plaque.

2. Narrowing Arteries: The atheroma plaque narrows the artery, restricting blood flow and raising blood pressure.

3. Increased Risk of Blockage: The narrowed, damaged artery wall increases the risk of two severe complications:

• Thrombosis: The rupture of the plaque can trigger the formation of a blood clot (thrombus), completely blocking the artery. This leads directly to a myocardial infarction (heart attack).
• Aneurysm: The damaged artery wall may weaken, leading to an abnormal bulge (aneurysm) which can burst (rupture), causing massive internal bleeding.

Risk Factors Associated with CHD

These factors are not causes in themselves, but they significantly increase the probability (risk) of developing CHD:

• Diet: A diet high in saturated fats and salt increases blood cholesterol levels and blood pressure.
• Blood Cholesterol: High levels of LDL ('bad') cholesterol contribute directly to atheroma formation.
• Cigarette Smoking: Nicotine increases heart rate and blood pressure, while carbon monoxide reduces the oxygen-carrying capacity of the blood, damaging artery walls.
• High Blood Pressure (Hypertension): High pressure damages the artery lining, making it more vulnerable to atheroma formation.

Did you know? Lifestyle changes, such as exercising regularly, maintaining a healthy weight, and quitting smoking, can dramatically reduce the risk of these conditions because they directly tackle factors like high blood pressure and high cholesterol.

Section 4: Cancer and Risk Factors (3.2.11.2)

Cancer is another major non-communicable disease. At its heart, cancer is a disease of uncontrolled cell division, often linked to changes in the genes that regulate the cell cycle (proto-oncogenes and tumour suppressor genes).

While some genetic mutations leading to cancer are inherited, most cancers result from mutations acquired during a lifetime due to exposure to environmental and lifestyle risk factors.

Lifestyle Risks Associated with Cancer

Specific lifestyle and environmental factors increase the risk of mutations occurring, leading to cancer:

• Smoking: Causes lung, throat, and bladder cancers. Smoke contains many carcinogens (cancer-causing agents).
• Diet: Poor diet lacking in fibre and high in processed foods is linked to certain digestive tract cancers.
• UV Exposure: Excessive exposure to ultraviolet radiation (e.g., sunlight) can damage DNA in skin cells, leading to skin cancer.
• Alcohol Consumption: Linked to liver and breast cancers.

The overall message here is that genes load the gun, but lifestyle pulls the trigger. You might have inherited a higher genetic risk, but lifestyle choices determine whether that risk manifests as disease.

Section 5: Analysing Data on Disease Risk (Skill Focus 3.2.1.2)

A crucial skill in this section is the ability to analyse, interpret, and evaluate data related to risk factors and disease incidence. Scientists often use large population studies (epidemiology) to find links between factors like diet or smoking and the probability of developing a disease.

Correlation vs. Causation

When analysing data, it is vital to remember the core biological principle: Correlation does not necessarily mean that there is a causal relationship.

• Correlation: This means two variables are statistically related. As one variable changes, the other tends to change as well. (Example: People who drink more coffee also tend to smoke.)
• Causation: This means one variable directly causes a change in the other. (Example: Smoking causes damage to arterial walls.)

When presented with data showing a correlation between a risk factor (like high sugar intake) and a disease (like CHD), you must evaluate whether the data proves direct cause, or whether other factors (confounding variables) might be involved.

How to Evaluate Data:

1. Sample Size: Is the population studied large enough to be representative?
2. Control Groups: Were there appropriate control groups for comparison?
3. Mechanism: Is there a clear biological mechanism (like atheroma formation) that explains the link? If yes, it strengthens the case for causation.
4. Replication: Have other studies found the same results?

Key Takeaway

Disease stems from three main areas: Pathogens (infectious agents like bacteria and viruses), Lifestyle (choices influencing non-communicable diseases like CHD and cancer), and Genes (predisposing factors that interact with lifestyle). Understanding these relationships is key to controlling disease incidence globally.