Communicable diseases

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

Bacterial diseases

  • 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)

Viral diseases

  • 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

    • Tobacco mosaic virus

Fungal diseases

  • Can be single-celled or multicellular.

  • Produces spores which spreads to other organisms.

  • Example:

    • Athlete’s foot

    • Rose black spot

Protist diseases

  • Single-celled eukaryotic organisms.

  • Often spread by vectors.

  • Example:

    • Malaria (spread by mosquitoes)

Disease

Pathogen Type

Transmission

Symptoms

Prevention / Control

Measles

Virus

Sneezing / coughing (droplets).

Fever and red skin rash.

Vaccination

HIV

Virus

Sexual contact or sharing needles.

Flu-like illness AIDS (damaged immune system).

Condoms / Avoid sharing needles / Antiretroviral drugs

TMV

Virus

Direct contact / vectors (insects).

Mosaic leaf pattern (stunted growth / less photosynthesis).

Remove infected plants / Wash tools

Salmonella

Bacteria

Contaminated food / unhygienic prep.

Fever, cramps, vomiting, diarrhoea.

Vaccinate poultry / Hygienic cooking

Gonorrhoea

Bacteria

Unprotected sexual contact.

Yellow/green discharge, painful urination.

Condoms / Antibiotics

Rose Black Spot

Fungus

Water or wind.

Black/purple spots on leaves (stunted growth).

Fungicides / Remove infected leaves

Malaria

Protist

Mosquito vector (bites).

Recurrent fevers (can be fatal).

Mosquito nets / Insect repellent / Drain stagnant water

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 and makes 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.

Antibiotics and painkillers

Antibiotics kill infective bacteria inside the human body, without harming body cells.

Specific antibiotics target specific 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

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

Preclinical testing: Using cells, tissues and live animals

Clinical testing: It is first tested on healthy volunteers with a low dose to ensure there are no harmful side effects.

The drugs are then tested on patients to find the most effective dose.

Single-blind test: (only the doctor knows whether the patient is receiving the drug)

double blind test: (neither the patient or doctor knows whether they are receiving the drug).

This removes any biases the doctor may have when they are recording the results.

The results then need to be peer reviewed by other scientists to check for repeatability.

  • The heart drug digitalis originates from foxgloves.

  • The painkiller aspirin originates from willow.

  • Penicillin was discovered by Alexander Fleming from the Penicillium mould.

Toxicity-Is it harmful

Efficacy-does it work?

Dose-What amount is safe and effective to give.