Welcome to Topic 5: Health, Disease, and the Development of Medicines!
In this chapter, we are going to explore what it actually means to be "healthy" and how different types of diseases affect us. We will look at how tiny organisms called pathogens make us sick, how our bodies (and even plants!) fight back, and how scientists develop amazing new medicines like vaccines and monoclonal antibodies to keep us safe. Don't worry if some of the long names look scary; we will break them down step-by-step!
1. What is Health?
When you think of health, you might just think it means "not being sick." But the World Health Organization (WHO) says it's much more than that.
Health is a state of complete physical, mental, and social well-being. It is not just the absence of disease.
Communicable vs. Non-communicable
Diseases are generally split into two groups:
- Communicable diseases: These can be passed from person to person (or animal to person). They are caused by pathogens like bacteria or viruses. Example: The flu or a cold.
- Non-communicable diseases: These cannot be passed between people. They are usually caused by genetics, lifestyle choices, or environmental factors. Example: Asthma or heart disease.
The Domino Effect of Disease
Sometimes, having one disease can make you more susceptible (likely to get) another. This is because some diseases weaken your immune system. For example, the HIV virus attacks white blood cells, making it much harder for the body to fight off other infections like tuberculosis (TB).
Quick Review: Health is physical, mental, and social. Communicable = catchy; Non-communicable = not catchy.
2. Pathogens and Common Infections
A pathogen is simply a "disease-causing organism." There are four main types you need to know:
- Viruses (e.g., HIV)
- Bacteria (e.g., Cholera)
- Fungi (e.g., Chalara ash dieback)
- Protists (e.g., Malaria)
Common Diseases to Remember
- Cholera: Caused by bacteria. Leads to severe diarrhoea. Spreads through contaminated water.
- Tuberculosis (TB): Caused by bacteria. Causes lung damage. It is airborne (spread through coughs/sneezes).
- Chalara ash dieback: A fungus that affects ash trees, causing leaf loss and bark lesions. It spreads through the air.
- Malaria: Caused by a protist. It damages the blood and liver. It is spread by animal vectors (mosquitoes).
- HIV: A virus that destroys white blood cells. This can lead to AIDS. Spread by body fluids.
- Stomach Ulcers: Caused by the Helicobacter bacteria. Spread by oral transmission (swallowing contaminated food/water).
- Ebola: A virus that causes haemorrhagic fever (internal bleeding). Spread through body fluids.
Did you know? A "vector" is just a biological taxi! In malaria, the mosquito is the vector because it carries the pathogen from one person to another without getting sick itself.
3. How Viruses Work (The Lytic and Lysogenic Pathways)
Viruses aren't technically "alive" because they need a host cell to reproduce. They do this in two ways:
The Lytic Pathway (The "Fast" Way)
1. The virus attaches to a host cell and injects its genetic material.
2. The cell is forced to make lots of new virus parts.
3. The virus parts assemble into new viruses.
4. The cell lyses (bursts open), releasing the viruses to infect more cells.
The Lysogenic Pathway (The "Sneaky" Way)
1. The virus injects its DNA into the host cell.
2. The viral DNA becomes part of the host cell's DNA.
3. Every time the host cell divides, it copies the viral DNA too!
4. Eventually, a trigger (like stress) causes the viral DNA to leave the host DNA and start the lytic pathway.
Key Takeaway: Lytic = burst and spread immediately. Lysogenic = hide in the DNA and wait.
4. Sexually Transmitted Infections (STIs)
STIs are spread through sexual contact. Two main ones to know are:
- Chlamydia: A bacterium. It often has no symptoms but can cause infertility. Prevention: Using a condom.
- HIV: A virus. Prevention: Using condoms, not sharing needles, and screening blood for transfusions.
5. Plant Defenses
Plants can't run away from pests, so they have built-in armor and weapons.
- Physical Barriers: The waxy leaf cuticle and cellulose cell walls act like a skin to keep pathogens out.
- Chemical Defenses: Plants produce chemicals to kill pests or microbes. We actually use some of these for human medicine, like aspirin (from willow trees) or quinine (from cinchona trees).
Detecting Plant Disease: Scientists can identify plant diseases in the field by looking for visible symptoms (like yellow leaves) or by distribution analysis (seeing if the disease is spreading by wind or soil). In the lab, they use diagnostic testing to find the specific pathogen.
6. Human Defenses
Your body has two "lines of defense":
First Line: Physical and Chemical Barriers
These try to stop pathogens from getting inside in the first place.
- Physical: Skin (a waterproof barrier), Mucus (sticky trap in the nose/throat), and Cilia (tiny hairs that sweep mucus away from lungs).
- Chemical: Lysozymes (enzymes in tears that kill bacteria) and Hydrochloric Acid (in the stomach to kill pathogens on food).
Second Line: The Specific Immune System
If a pathogen gets past the barriers, your white blood cells take over:
1. Every pathogen has unique molecules on its surface called antigens.
2. When white blood cells (lymphocytes) see a new antigen, they produce antibodies.
3. Antibodies are specific shapes that "lock" onto the antigens to help destroy the pathogen.
4. The body also makes memory lymphocytes. These stay in the blood for years.
Analogy: Think of antibodies like a specialized key for a specific lock. Once your body knows how to make that key, it keeps the blueprint (memory cells) so it can make them much faster if the pathogen ever returns!
7. Immunisation (Vaccination)
Vaccines "trick" your body into thinking it's sick without actually making you ill.
How it works: A dead or inactive form of the pathogen is injected. Your immune system creates antibodies and memory lymphocytes. If you ever catch the real disease, your body kills it before you even feel sick.
Herd Immunity: When a large part of the population is vaccinated, the disease cannot spread easily, which protects people who can't be vaccinated (like very sick people or newborn babies).
8. Antibiotics
Antibiotics are medicines that kill bacteria. They work by inhibiting cell processes in the bacterium.
Important: Antibiotics DO NOT work on viruses. This is a common mistake! Since viruses live inside human cells, it is very hard to kill them without damaging your own body cells.
9. Developing New Medicines
New drugs have to be tested thoroughly to make sure they are safe and actually work. The stages are:
- Discovery: Finding a potential new drug (often from plants).
- Preclinical Testing: Testing the drug on cells and tissues in the lab, and then on animals to check for toxicity.
- Clinical Testing: Testing on human volunteers. First, on healthy people (to check for safety), then on people with the disease (to find the right dose).
10. Monoclonal Antibodies (Higher Tier)
Monoclonal antibodies are identical copies of one specific antibody. We make them using hybridoma cells.
How to make them:
1. Take a lymphocyte (which makes the right antibody but doesn't divide).
2. Fuse it with a myeloma cell (a cancer cell that divides very quickly but doesn't make antibodies).
3. This creates a hybridoma cell. This "super-cell" can both make antibodies AND divide forever!
4. We collect the antibodies they produce.
Uses:
- Pregnancy tests: They bind to a hormone found only in pregnant women's urine.
- Diagnosis: They can be made to stick to cancer cells or blood clots so we can see them on a scan.
- Treatment: They can carry drugs directly to cancer cells without harming healthy cells.
11. Lifestyle and Non-Communicable Disease
Many diseases are caused by how we live. Factors include:
- Diet and Exercise: A poor diet can lead to obesity or malnutrition.
- Alcohol: Can cause liver disease (cirrhosis).
- Smoking: A major risk factor for cardiovascular disease (heart/lung disease).
Math Skills: BMI
We use the Body Mass Index (BMI) to see if a person is a healthy weight:
\[ BMI = \frac{mass (kg)}{(height (m))^2} \]
Example: If a person weighs 70kg and is 1.75m tall, their BMI is \( 70 \div (1.75 \times 1.75) = 22.86 \).
Treating Cardiovascular Disease (CVD)
- Lifestyle changes: Better diet, more exercise, stopping smoking.
- Medication: Drugs to lower blood pressure or cholesterol.
- Surgical procedures: Stents to open arteries or heart bypass surgery.
Final Key Takeaway: Health is a balance of physical and mental factors. We protect ourselves through natural barriers, immune responses, and the clever use of medicines like vaccines and antibiotics!