Welcome to "Infection and Response"
In this chapter, we are going to explore the world of communicable diseases. These are illnesses that can be passed from one person (or organism) to another. Think of them as "catchable" diseases. We will learn how "germs" make us sick, how our body fights back, and how medicine helps us win the war against infection.
4.3.1 Communicable (Infectious) Diseases
A pathogen is just a scientific word for a microorganism that causes disease. There are four main types of pathogens you need to know:
• Viruses (like the flu)
• Bacteria (like Salmonella)
• Protists (like the ones that cause Malaria)
• Fungi (like Athlete’s foot)
How do they spread?
Pathogens are like tiny hitchhikers. They can travel by air (coughing/sneezing), water (drinking dirty water), or direct contact (touching someone or a surface).
Bacteria are tiny living cells. Once inside you, they divide very quickly. They make you feel ill by producing toxins (poisons) that damage your cells and tissues.
Viruses are even smaller. They aren't cells. They move inside your cells and use your cell’s "machinery" to make copies of themselves. Eventually, the cell bursts, releasing more viruses. This cell damage is what makes you feel sick.
Quick Review: Pathogens = Germs. Bacteria make toxins; Viruses live inside cells and burst them.
4.3.1.2 Specific Viral Diseases
1. Measles: This is spread by droplets from sneezes and coughs. Symptoms include a fever and a red skin rash. It can be very serious, which is why most children are vaccinated against it.
2. HIV: This starts with a flu-like illness. If not controlled, it attacks the immune cells. When the immune system is too damaged to fight other infections, it is called AIDS. It is spread by sexual contact or sharing needles.
3. Tobacco Mosaic Virus (TMV): This affects plants (like tomatoes). It creates a "mosaic" pattern of discolouration on leaves. This is bad because the plant can't do photosynthesis as well, so it doesn't grow properly.
4.3.1.3 Specific Bacterial Diseases
1. Salmonella: This is food poisoning. You get it by eating food contaminated with bacteria (like undercooked chicken). Symptoms include fever, cramps, vomiting, and diarrhoea. In the UK, poultry (chickens) are vaccinated against it to stop the spread.
2. Gonorrhoea: This is a sexually transmitted disease (STD). Symptoms include a thick yellow/green discharge and pain when urinating. It used to be easy to treat with penicillin (an antibiotic), but many strains are now resistant. We prevent it with antibiotics or barrier methods (condoms).
4.3.1.4 Fungal and Protist Diseases
Rose Black Spot (Fungal): Purple or black spots appear on leaves. The leaves turn yellow and fall off. This reduces photosynthesis, so the plant doesn't grow well. It spreads by water or wind. We treat it with fungicides or by removing the affected leaves.
Malaria (Protist): This is caused by a protist. The mosquito is the "vector" (it carries the disease but doesn't get sick). When the mosquito bites a human, it passes the protist into the blood. It causes repeated fevers and can be fatal. We stop it by stopping mosquitos from breeding and using mosquito nets.
Key Takeaway: Different diseases have different symptoms, but they all depend on a host (you or a plant) to survive and reproduce.
4.3.1.6 Your Body's Defence System
Don't worry if this seems like a lot to fight—your body is like a fortress! You have two layers of defence.
Layer 1: Non-specific Defences (The Barriers)
These try to keep the pathogens out of your body:
• Skin: A physical barrier that also produces antimicrobial substances.
• Nose: Hairs and mucus trap particles.
• Trachea and Bronchi: Produce mucus to trap germs and have "cilia" (tiny hairs) to waft the mucus up to the throat to be swallowed.
• Stomach: Produces hydrochloric acid to kill pathogens in food.
Layer 2: The Immune System (The Army)
If pathogens get inside, your white blood cells take over. They have three main jobs (Remember P.A.A.):
1. Phagocytosis: They literally engulf (eat) and digest the pathogens.
2. Antibody production: They produce proteins called antibodies that lock onto specific pathogens and kill them. If you get the same germ again, your body remembers how to make these antibodies fast!
3. Antitoxin production: They make antitoxins to cancel out the toxins (poisons) produced by bacteria.
Memory Aid: White blood cells are the "Security Guards." They Eat the bad guys, Tag them for destruction, and Clean up the poison.
4.3.1.7 Vaccination
A vaccination involves putting small amounts of dead or inactive pathogens into your body. They are "harmless," but they "trick" your white blood cells into making antibodies. If the real, "live" disease ever enters your body, your immune system is already trained to destroy it instantly.
4.3.1.8 Antibiotics and Painkillers
It is very important to know the difference between these two!
• Painkillers: These (like Aspirin) just treat the symptoms. They make you feel better, but they do not kill the pathogen.
• Antibiotics: These (like Penicillin) are medicines that kill bacteria. They have saved millions of lives.
BIG WARNING: Antibiotics cannot kill viruses. This is because viruses live inside your body cells; it's very hard to kill the virus without killing the cell too.
Common Mistake: Never say "Antibiotics kill viruses." They ONLY work on bacteria!
4.3.1.9 Developing New Drugs
In the past, many drugs came from plants. For example, Digitalis (heart drug) comes from foxgloves, and Aspirin comes from willow trees. Nowadays, most drugs are made in labs, but we must test them carefully.
The testing steps:
1. Preclinical testing: Done in a lab using cells, tissues, and live animals. We check for toxicity (is it poisonous?).
2. Clinical trials: Done on healthy volunteers first to check for safety at low doses. Then, it is tested on patients to find the best dose and check efficacy (does it actually work?).
3. Double-blind trials: Some patients get the drug, and some get a placebo (a fake pill). Neither the doctor nor the patient knows who has the real drug until the end. This stops bias.
4.3.2 Monoclonal Antibodies (Higher Tier Only)
Monoclonal antibodies are produced from a single clone of cells. They are special because they are designed to target only one specific protein (antigen) in your body. This makes them like "smart bombs" that only hit one target.
How are they made?
1. A mouse is stimulated to make a specific antibody.
2. We take the lymphocytes (white blood cells) from the mouse.
3. We fuse the lymphocyte with a tumour cell to create a hybridoma cell.
4. This hybridoma cell can both divide (like a tumour) and make antibodies (like a lymphocyte).
5. We clone this cell to make a huge amount of the same antibody.
Uses: They are used in pregnancy tests, to measure hormone levels in blood, and to treat cancer by carrying drugs directly to tumour cells without harming healthy cells.
4.3.3 Plant Disease (Biology Only)
Plants get sick too! You can spot a sick plant by looking for stunted growth, spots, rot, or discolouration. They can also suffer from ion deficiencies:
• Nitrate deficiency: Needed for protein synthesis, so the plant has stunted growth.
• Magnesium deficiency: Needed to make chlorophyll. Without it, the leaves turn yellow (this is called chlorosis).
Plant Defences
Plants can't run away, so they have clever ways to protect themselves:
• Physical: Cellulose cell walls, a tough waxy cuticle on leaves, and bark on trees act as "skin."
• Chemical: Some plants produce antibacterial chemicals or poisons to stop animals from eating them.
• Mechanical: Thorns and hairs stop animals. Some leaves droop or curl when touched to shake off insects.
Final Review: Communicable diseases are caused by pathogens. Our body uses barriers and white blood cells to fight them. We use vaccinations to prevent them and antibiotics to kill bacteria. Drugs must be tested in clinical trials for safety and efficacy before they are used.