Communicable diseases
What Are Communicable Diseases?
Communicable (infectious) diseases are diseases caused by pathogens.
A pathogen is a microorganism that causes diseases
They can be spread between organisms, in the water and in the air.
Pathogens include:
Bacteria
Viruses
Fungi
Protists
Pathogens and the Diseases They Cause
Bacteria
Microscopic single-celled organisms.
Can reproduce rapidly through a process called binary fission inside the body.
Produce toxins that damage cells.
Examples:
Salmonella – food poisoning
Gonorrhoea – sexually transmitted infection (STI)
Viruses
Not living cells.
Much smaller than bacteria.
Viruses reproduce inside host cells by using the cell’s machinery, which damages or destroys the cell when new viruses are released.
Examples:
Measles
HIV
Influenza
Fungi
Can be single-celled or multicellular.
Produces spores which spreads to other organisms.
Example:
Athlete’s foot
Protists
Single-celled eukaryotic organisms.
Often spread by vectors.
Example:
Malaria (spread by mosquitoes)
How Communicable Diseases Are Spread
Methods of Transmission:
Direct contact (touching, sexual contact)
Indirect contact (contaminated surfaces, food, water)
Droplets in the air (coughing, sneezing)
Vectors (organisms that carry pathogens, e.g. mosquitoes)
Disease | Type of Pathogen | How It Is Spread | Key Symptoms / Effects | Control / Prevention |
|---|
Measles | Virus | Droplets from coughs and sneezes | Fever, red skin rash, can cause serious complications | Vaccination |
HIV | Virus | Sexual contact or exchange of body fluids | Attacks immune system, can lead to AIDS | No cure; antiretroviral drugs slow progression |
Tobacco Mosaic Virus (TMV) | Virus | Contact between infected plants | Mosaic pattern on leaves, reduced photosynthesis and growth | Remove infected plants, good hygiene |
Salmonella | Bacteria | Contaminated food (especially poultry) | Fever, abdominal cramps, vomiting | Good hygiene, vaccinating poultry |
Gonorrhoea | Bacteria | Sexual contact (STI) | Pain when urinating, thick yellow/green discharge | Antibiotics (resistance increasing) |
Malaria | Protist | Female Anopheles mosquito (vector) | Recurrent fever | Insecticide-treated bed nets, preventing mosquito breeding |
The non specific defence system is to prevent pathogens from entering the body.
Human Defence Systems
Physical Barriers
Skin – acts as a barrier to pathogens.
Mucus in the nose and airways traps pathogens.
Cilia move mucus out of the lungs.
Chemical Defences
Hydrochloric acid in the stomach kills pathogens.
Lysozyme enzymes in tears and saliva.
The Immune System
The immune system destroys pathogens and any toxins they produce.
The immune system protects us from the same type of pathogen that invades us in the future.
White Blood Cell | Function / Role |
|---|---|
Phagocytes | Phagocytes are attracted to the area of infection The phagocytes surrounds the pathogen and ingests it. Enzymes that digest and destroy the pathogen are released. |
lymphocytes | Lymphocytes fight pathogen in two ways: |
Antibodies | Lymphocytes produce antibodies that target and help to destroy pathogens by binding to pathogens antigens on the pathogens surface. This means that they are extremely specific |
Antitoxins | Lymphocytes produce antitoxins which binds to toxic molecules produced by pathogens. This neutralises the toxins |
Vaccination
Vaccination involves injecting small quantities of dead or inactive forms of a pathogen into the body.
This stimulates lymphocytes, which divide by mitosis to produce plasma cells that make the correct antibodies and memory cells.
If the same pathogen enters the body again, the memory cells recognise it and cause a rapid production of antibodies, preventing infection and providing long-term immunity.
Herd immunity
If a large proportion of a population is vaccinated, against a disease, the disease is less likely to spread, even if there are some unvaccinated individuals.
The unvaccinated person cannot catch the disease because no-one around them can pass the pathogen around.
Type | Description |
|---|---|
Live attenuated | Contain weakened forms of the pathogen |
Inactivated | Contain dead pathogens |
Subunit | Contain only parts of the pathogen (e.g. antigens or proteins) |
mRNA | Contain instructions for cells to produce a pathogen protein |
Advantage | Explanation |
|---|---|
Individual protection | Reduces risk of becoming seriously ill |
Herd immunity | Reduces spread and protects vulnerable people |
Prevents epidemics | Stops diseases spreading rapidly |
Long-term immunity | Memory cells remain in the body |
Limitation | Explanation |
|---|---|
Side effects | Mild effects such as fever or swelling |
Not suitable for everyone | Some people have weakened immune systems |
Not always 100% effective | Some vaccinated people may still get the disease |
Pathogens mutate | Changes can reduce vaccine effectiveness |
Antibiotics and painkillers
Antibiotics kill infective bacteria inside the human body, without harming body cells.
Antibiotics target bacterial cells, for example by:
Damaging the bacterial cell wall
Interfering with bacterial protein synthesis
Antibiotics do NOT work against viral infections because:
Viruses live inside body cells so antibiotics being used can damage body tissues
Viruses have a different structure to bacteria
Specific bacteria needs to be treated by specific antibiotics.
Certain antibiotics were no longer effective against certain bacteria as antibiotics have been overused. This is because the bacteria has been evolved so that they were no longer killed by the antibiotic.
Reducing Antibiotic Resistance
Only use antibiotics when necessary
Always complete the full course
Avoid using antibiotics for viral infections
Painkillers treat the symptom of the disease by relieving pain
How painkillers work
They block pain signals from nerves to the brain.
Some reduce inflammation.
Discovery and development of drugs
Drug sources: Traditionally from plants & microorganisms (e.g. aspirin from willow, penicillin from mould).
Modern development: Computer modelling designs drugs to fit receptors; drugs are synthesised and optimised.
Preclinical testing: On cells/tissues & animals → tests
Toxicity-Is it harmful
Efficacy-does it work?
Dose-What amount is safe and effective to give.
Clinical trials:
Healthy volunteers → safety & dosage
Patients → effectiveness & side effects
Double blind trials: Neither doctor nor patient knows who gets the drug or placebo (known to have no effect)→ reduces bias incase the doctors pay closer attention to the person receiving the actual drug.
Peer review: Other scientists check and repeat results → improves validity & reliability.