Communicable Diseases and Pathogens
What Are Communicable Diseases?
Communicable diseases (infectious diseases) are caused by pathogens — microorganisms that enter the body and cause illness. They can be spread from person to person or from the environment.
Types of Pathogen
| Pathogen type | Size | Examples of diseases | How they make you ill |
|---|---|---|---|
| Bacteria | Very small (~1-5 μm) | Salmonella, gonorrhoea, tuberculosis (TB) | Reproduce rapidly; release toxins that damage cells and tissues |
| Viruses | Extremely small (~20-300 nm) | HIV, measles, TMV (tobacco mosaic virus), influenza | Invade cells and hijack cell machinery to replicate; the host cell bursts, releasing new viruses and destroying the cell |
| Fungi | Varies | Athlete's foot, rose black spot | Hyphae grow into tissues; produce spores for reproduction |
| Protists | Single-celled eukaryotes | Malaria | Often transferred by a vector (another organism); damage cells and tissues |
How Pathogens Spread
| Method of spread | Examples |
|---|---|
| Airborne (droplets) | Influenza, TB, measles — when an infected person coughs or sneezes |
| Direct contact | Athlete's foot (touching contaminated surfaces), gonorrhoea (sexual contact) |
| Water | Cholera — contaminated drinking water |
| Vectors | Malaria — spread by mosquitoes (Anopheles); the mosquito is the vector |
| Food | Salmonella — undercooked food, especially chicken and eggs |
| Body fluids | HIV — through blood, sexual fluids, breast milk, sharing needles |
Specific Diseases You Must Know
Measles (virus)
- Spread by airborne droplets (coughing/sneezing)
- Symptoms: fever, red skin rash, can lead to pneumonia or encephalitis (brain inflammation) — potentially fatal
- Prevention: MMR vaccine; isolation of infected individuals
HIV (Human Immunodeficiency Virus)
- Spread through body fluids: sexual contact, sharing needles, mother to child during birth or breastfeeding
- Initially causes flu-like symptoms, then the virus attacks lymphocytes (white blood cells)
- Late stage: the immune system is so weakened it cannot fight infections → AIDS (Acquired Immune Deficiency Syndrome)
- Controlled (not cured) with antiretroviral drugs that slow viral replication
- Prevention: condoms, screening blood donations, not sharing needles
Salmonella (bacterium)
- Spread by eating contaminated food (especially poultry and eggs)
- Bacteria produce toxins causing fever, vomiting, diarrhoea, abdominal cramps
- Prevention: thorough cooking (above 75°C), good food hygiene, vaccinating poultry
Gonorrhoea (bacterium)
- A sexually transmitted infection (STI)
- Symptoms: thick yellow/green discharge, pain when urinating; can be symptomless
- Treated with antibiotics, but some strains are becoming antibiotic-resistant
- Prevention: using condoms (barrier method); contact tracing
Tobacco Mosaic Virus (TMV) (virus — plant disease)
- Affects many plant species, especially tobacco and tomato plants
- Causes a distinctive mosaic pattern of discolouration on leaves (light and dark patches)
- This reduces the area of chlorophyll, reducing photosynthesis → stunted growth
- Spread by direct contact between plants, contaminated tools, or vectors
Rose Black Spot (fungus — plant disease)
- Causes purple/black spots on leaves, which then turn yellow and drop
- Fewer leaves = less photosynthesis = reduced growth
- Spread by water (rain splash) and wind (spores)
- Treated with fungicides or by removing and destroying affected leaves
Malaria (protist — Plasmodium)
- Spread by the bite of an infected female Anopheles mosquito (the vector)
- The protist enters the blood and destroys red blood cells
- Symptoms: recurring episodes of fever, chills, sweating; can be fatal
- Prevention: mosquito nets, insecticides, draining stagnant water (breeding sites), antimalarial drugs
Reducing the Spread of Disease
- Hygiene — handwashing, safe food preparation
- Isolation of infected individuals
- Vaccination programmes
- Destroying vectors — insecticides, mosquito nets, draining standing water
- Contraception (barrier methods like condoms for STIs)
Exam Tips
- You must know the specific diseases listed above — pathogen type, transmission, symptoms, and prevention for each
- TMV and rose black spot are plant diseases — do not confuse them with human diseases
- Malaria is caused by a protist, not a bacterium or virus — the mosquito is the vector, not the pathogen