Animal Pathogens and Diseases

This lesson covers:

  1. The different types of pathogen 
  2. How pathogens are transmitted 
  3. Examples of communicable diseases 

Pathogens 

Communicable (or infectious) diseases are those that can be passed from one organism to another. They are caused by microorganisms known as pathogens.


There are four types of pathogen you need to know: 

Illustration showing a bacterium and a virus with labels.
Illustration showing protoctists and fungi, two types of pathogens.
  1. Bacteria - Produce toxins that damage body cells. 
  2. Viruses - Use host cells to replicate before bursting out and destroying cells.
  3. Protoctists (protists) - Take over cells and break them open.
  4. Fungi - Digest living cells to destroy them. Some also produce toxins.

Modes of transmission

Communicable diseases can be passed from one organism to another via direct or indirect transmission. 

Direct transmission 

Pathogens can be transferred directly from one organism to another.


This transfer can take place in several ways: 

  • Direct contact - Skin-to skin contact, kissing, or sexual intercourse.
  • Airborne droplets - Coughing or sneezing tiny droplets of mucus or saliva onto someone.  

Indirect transmission 

Communicable diseases can also be passed from one organism to another using an intermediate (e.g. water or another organism).


This transfer can take place in several ways:

  • Food and drinking water - Ingestion of contaminated food or water can cause disease.
  • Vectors - These transmit pathogens from one host to another (e.g. mosquitoes transmit malaria).
  • Contaminated objects - Pathogens from infected individuals can live on objects for a short time and infect others.

The risk of communicable disease is increased by certain factors:

  • Living conditions - Overcrowded areas increase the risk of direct transmission.
  • Climate - For example, warmer temperatures allow mosquitoes to breed and transmit malaria. 
  • Social factors - For example, a lack of health education and healthcare systems increase the risk of communicable disease.

Examples of communicable diseases 

You need to be able to describe the cause and transmission of the following communicable diseases:

DiseasePathogenOrganisms affectedEffect on the bodyMode of transmission
Tuberculosis (TB)BacteriumHumans, cows, pigs, and badgersDamages the lungs and suppresses the immune systemAirborne droplets, Contaminated food and drink
Bacterial meningitisBacteriumYoung children and teenagersDamages membranes of the brain and can cause blood poisoningAirborne droplets
HIV/AIDSVirusHumans and non-human primatesGradually destroys the immune systemExchange of bodily fluids
InfluenzaVirusMammals including humans, pigs, and birdsKills ciliated epithelial cells in the gas exchange systemAirborne droplets, Contaminated objects
Athlete’s footFungusHumansCauses cracking and scaling of skin between the toesContaminated objects
RingwormFungusMammals including cattle, dogs, cats, and humansCauses a circular, red rashDirect contact
MalariaProtoctistHumans, other mammals, reptiles, and birdsDamages the red blood cells, liver, and brainVector (Anopheles mosquito)