How diseases spread

• Communicable diseases are caused by pathogens. 

  Pathogens - microorganisms such as bacteria, virus, fungi, and protists that cause diseases in living organisms (humans, animals, and plants).
• 4 TYPES OF PATHOGENS

  • Bacteria
  • Virus
  • Protist
  • Fungi
• Bacteria 

  • Very small cells (about 1/100th the size of your body cells), which can reproduce  rapidly. 
  • They make you feel ill by producing toxins (poisons) that damage your cells and tissues. 
• Virus

  • These are not cells. Non-living things that can live anywhere.
  • They’re really tiny, about 1/100th the size of a bacterium. 
  • They replicate themselves inside the infected organism’s cells. 
  • These cells then burst, releasing the viruses.  
  • Needs a host to reproduce.
  • Smallest.
• Protist 

  • These are eukaryotic, usually single-celled and vary in size.  
  • Needs a vector to transmit diseases or infections.
• Fungi

  • Some fungi are single-celled while others have a body, which is made up of thread-like structures called hyphae.
  • These hyphae can grow and penetrate human skin and the surface of plants,  causing diseases. 
  • They can also produce spores, which can be spread to other plants and animals. 
• COMMUNICABLE DISEASES ARE TRANSMITTED IN DIFFERENT WAYS

  • Water
  • Air
  • On surfaces
  • Body fluids
  • Animal vectors
  • Soil
  • Food
• Water

  • Some pathogens can be picked up by drinking or bathing in dirty water.

   Example:

  Cholera - is a bacterial infection that causes diarrhoea and dehydration. 

  • It’s spread when drinking water is contaminated with the diarrhoea  of other sufferers.
• Air

  • Some pathogens are carried in the air. 

  Example:

  Hymenoscyphus fraxineus - is a fungus which infects ash trees and causes Chalara ash dieback disease. 

  • It is transmitted from infected plants, through the air, by the wind. 
  • Airborne pathogens can be carried in droplets produced when you cough or sneeze — so other people can breathe them in. E.g. the influenza virus that causes flu is spread this way.  
• Pathogens

  Microorganisms that can cause disease
• Some pathogens can be picked up by touching contaminated surfaces
• Tobacco mosaic disease


  • Affects many species of plants, e.g. tomatoes
  • Caused by a virus called tobacco mosaic virus (TMV)
  • Makes the leaves of plants mottled and discoloured
  • Discolouration means the plant can't photosynthesise as well, so the virus affects growth
  • Spread when infected leaves rub against healthy leaves
• Athlete's foot


  • A fungal disease which affects humans
  • Makes skin on the feet itch and flake off
  • Most commonly spread by touching the same things as an infected person, e.g. shower floors and towels
• Body fluids

  Fluids in the body such as blood, breast milk, and semen
• Some pathogens are spread by body fluids such as blood (e.g. by sharing needles to inject drugs or by contaminated blood transfusions), breast milk (through breastfeeding) and semen (through unprotected sex â€" diseases that are spread through sexual contact are known as sexually transmitted infections or STIs)
• HIV


  A virus spread by exchanging body fluids
• HIV infection


  1. Initially causes flu-like symptoms for a few weeks

  2. After that, the person doesn't usually experience any symptoms for several years

  3. Virus enters the lymph nodes and attacks the immune cells

  4. If the immune system isn't working properly, it can't cope with other infections or cancers

  5. At this stage, the virus is known as late-stage HIV or AIDS
• Animal vectors

  • Animals that spread disease are called vectors.

  Example:

  • Rabies
  • Malaria is caused by a protist. Part of the malarial protist’s life cycle takes place inside a mosquito. 
  • Mosquitoes act as vectors — they pick up the malarial protist when they feed on an infected animal.
  • Every time the mosquito feeds on another animal, it infects it by inserting the protist into the animal’s blood vessels.
  •  Malaria causes repeating episodes of fever. It can be fatal.  
• Soil 

  • Some pathogens can live in the soil, so plants in the contaminated soil may be infected. 

  Example:

  • Agrobacterium tumefaciens - causes crown gall disease, is able to live freely in some soils and on the roots of some plants.
  • If the bacteria enter a plant, they can cause growths or tumours called galls on roots,  stems and branches. 
  • The galls can damage the plant tissue, restricting the flow of water through the plant.  
  • This causes the plant to become weaker and it may eventually die.  
• Food

  • Some pathogens are picked up by eating contaminated food.

  Example:

  • Salmonella bacteria are found in some foods,  e.g. raw meat. 
  • If these foods are kept too long or not cooked properly the bacteria can cause food poisoning.