# Communicable Diseases and Pathogens
Communicable diseases (also called infectious diseases) are caused by pathogens — microorganisms that invade the body and cause illness. These diseases can be spread from one organism to another. Understanding the types of pathogens, how they spread, and specific disease examples is essential for GCSE Biology.
1. What Are Pathogens?
A pathogen is a microorganism that causes disease. There are four main types:
| Type | Description | Examples of Diseases |
|---|---|---|
| Bacteria | Single-celled prokaryotes; reproduce rapidly by binary fission; release toxins that damage cells and tissues | Salmonella, tuberculosis (TB), gonorrhoea |
| Viruses | Much smaller than bacteria; not truly living; reproduce inside host cells, destroying them as new viruses burst out | Influenza (flu), HIV, measles, tobacco mosaic virus (TMV) |
| Fungi | Can be single-celled or multicellular; send out threads (hyphae) that penetrate tissues | Athlete's foot, rose black spot |
| Protists | Eukaryotic organisms, often single-celled; some are parasites that live in or on other organisms | Malaria (caused by Plasmodium, spread by mosquitoes) |
2. How Pathogens Spread
Pathogens can be transmitted in several ways:
| Route of Transmission | Description | Examples |
|---|---|---|
| Direct contact | Touching an infected person or their bodily fluids | Gonorrhoea (sexually transmitted), athlete's foot (skin contact) |
| Airborne (droplet) | Inhaling tiny droplets expelled by coughing or sneezing | Influenza, measles, tuberculosis |
| Waterborne | Drinking or using contaminated water | Cholera, dysentery |
| Vector-borne | Spread by another organism (a vector), such as an insect | Malaria (mosquito vector) |
| Foodborne | Eating contaminated or undercooked food | Salmonella (from contaminated poultry) |
3. Specific Disease Examples
You need to know these diseases for GCSE:
Measles (Virus)
- Spread: Airborne — droplets from coughs and sneezes (highly contagious)
- Symptoms: Fever, red skin rash, runny nose, cough
- Complications: Pneumonia, encephalitis (brain inflammation) — can be fatal in young children
- Prevention: MMR vaccine (measles, mumps, rubella)
HIV (Virus)
- Spread: Sexual contact, sharing needles, mother to child (breast milk or birth)
- Symptoms: Initially flu-like symptoms; over years, the virus attacks T-helper lymphocytes (immune cells)
- AIDS: If untreated, HIV leads to AIDS (Acquired Immune Deficiency Syndrome) — the immune system is so weakened that the body cannot fight off other infections
- Treatment: Antiretroviral drugs control the virus (not a cure); barrier contraception reduces transmission
Tobacco Mosaic Virus — TMV (Virus) — Plant Disease
- Affects: Many plants, especially tobacco and tomato plants
- Symptoms: Mosaic pattern of discolouration on leaves (patches of yellow/light green)
- Effect: Reduces the area of green leaf available for photosynthesis → reduced growth
- Spread: Direct contact, contaminated tools, insects
Salmonella (Bacteria)
- Spread: Eating contaminated food, especially undercooked poultry and eggs
- Symptoms: Fever, abdominal cramps, vomiting, diarrhoea
- Cause of illness: Bacteria produce toxins in the gut
- Prevention: Proper cooking of food; good hygiene; in the UK, poultry are vaccinated against Salmonella
Gonorrhoea (Bacteria) — Sexually Transmitted Infection (STI)
- Spread: Unprotected sexual contact
- Symptoms: Thick yellow/green discharge, pain when urinating; can be asymptomatic
- Treatment: Antibiotics — but many strains are becoming antibiotic-resistant
- Prevention: Barrier contraception (condoms)
Tuberculosis — TB (Bacteria)
- Spread: Airborne (droplet infection from coughing)
- Symptoms: Persistent cough (sometimes with blood), weight loss, fever, night sweats
- Treatment: Long course of antibiotics (6+ months)
- Prevention: BCG vaccine; isolation of infected individuals
Rose Black Spot (Fungus) — Plant Disease
- Affects: Rose plants
- Symptoms: Purple/black spots on leaves; leaves turn yellow and drop off
- Effect: Loss of leaves reduces photosynthesis → reduced growth
- Spread: Water (rain splash) and wind (spores)
- Treatment: Fungicides; removing and destroying infected leaves
Malaria (Protist)
- Caused by: Plasmodium — a protist parasite
- Spread: Through the bite of an infected female Anopheles mosquito (the vector)
- Symptoms: Recurring episodes of fever, chills, sweating, headache
- Prevention: Mosquito nets, insect repellent, draining stagnant water (breeding sites), antimalarial drugs
- Cycle: Mosquito bites infected person → Plasmodium enters mosquito → mosquito bites another person → Plasmodium enters their blood → infects liver and red blood cells
4. How the Body Prevents Entry of Pathogens
The body has several non-specific defences (they work against all pathogens):
| Defence | How It Works |
|---|---|
| Skin | A physical barrier that prevents pathogens from entering |
| Nose hairs and mucus | Trap pathogens in the airways |
| Trachea and bronchi | Lined with ciliated epithelial cells and mucus-producing goblet cells; cilia sweep mucus (and trapped pathogens) up to the throat to be swallowed |
| Stomach acid | Hydrochloric acid (pH ~2) kills many pathogens in food and swallowed mucus |
| Tears | Contain lysozyme, an enzyme that destroys bacterial cell walls |
| Wound clotting | Platelets cause blood to clot, sealing wounds and preventing pathogen entry |
5. Reducing the Spread of Disease
| Method | Examples |
|---|---|
| Hygiene | Handwashing, covering mouth when coughing/sneezing, disinfecting surfaces |
| Vaccination | MMR, BCG, flu vaccine — stimulates immunity before infection |
| Isolation | Keeping infected individuals away from others (quarantine) |
| Safe food preparation | Cooking food thoroughly, avoiding cross-contamination |
| Vector control | Using mosquito nets, insecticides, draining stagnant water |
| Barrier contraception | Condoms — prevent STIs like gonorrhoea and HIV |
Worked Example
Question: Describe how malaria is spread and suggest two methods to reduce its spread. (4 marks)
Solution:
Malaria is caused by a protist called Plasmodium. It is spread by the bite of an infected female Anopheles mosquito, which acts as a vector. When the mosquito bites a person, it injects the Plasmodium parasites into the blood.
Two methods to reduce its spread:
- Mosquito nets — sleeping under insecticide-treated nets prevents mosquitoes from biting at night when they are most active.
- Draining stagnant water — mosquitoes breed in still water, so removing these breeding sites reduces the mosquito population.
Practice Questions
- Define the term 'pathogen'. (1 mark)
- Name the four types of pathogen and give one disease example for each. (4 marks)
- Explain how the body's non-specific defences prevent pathogen entry. Give three examples. (6 marks)
- Describe the symptoms and spread of gonorrhoea. Explain why it is becoming harder to treat. (4 marks)
- Compare how TB and malaria are spread and prevented. (4 marks)
Answers
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Exam Tips
- Know at least one disease example for each pathogen type (bacteria, virus, fungus, protist).
- Be precise: mosquitoes don't "carry" malaria — they are vectors that transmit the Plasmodium protist.
- For plant diseases, focus on the effect on photosynthesis and growth.
- The question "How does the body prevent pathogen entry?" asks about non-specific defences (skin, acid, mucus, clotting), NOT the immune response.
Frequently Asked Questions
Are antibiotics effective against viruses?
No. Antibiotics only work against bacteria. Viruses live inside host cells and are not affected by antibiotics. Antiviral drugs exist for some viral infections (e.g., antiretrovirals for HIV), but they don't kill the virus — they slow its replication.
Why is antibiotic resistance a problem?
When bacteria develop resistance to antibiotics, infections become harder to treat. Resistant bacteria survive and reproduce, spreading the resistance. Overuse of antibiotics accelerates this process, potentially leading to "superbugs" that no current antibiotics can kill.
Is malaria contagious between people?
Not directly. Malaria is not spread by person-to-person contact, coughing, or sneezing. It is only spread through mosquito bites (or, rarely, through blood transfusions or shared needles).
Summary
- Pathogens are disease-causing microorganisms: bacteria, viruses, fungi, and protists.
- Diseases spread by direct contact, airborne droplets, water, food, or vectors.
- Key diseases: measles (virus), HIV (virus), Salmonella (bacteria), gonorrhoea (bacteria), TB (bacteria), malaria (protist), rose black spot (fungus), TMV (virus).
- The body's non-specific defences include skin, mucus and cilia, stomach acid, tears, and blood clotting.
- Spread is reduced by hygiene, vaccination, isolation, and vector control.