Communicable Diseases

Diseases are abnormal conditions that negatively affect a body’s cells, organs, body or tissue. Some of these effects are caused by pathogens. The diseases caused by pathogens are called communicable diseases.

  • Communicable diseases are transferred from one person to another


Pathogens are microorganisms that cause disease. These include viruses, bacteria, protists and fungi. All organisms can be affected by viruses, even bacteria can even be affected by certain types of viruses.

The four main types of pathogens are shown in the table below:

PathogenDescriptionExamples in animals
VirusesThey require a host to survive and reproduce.

Once the virus penetrates the host cell, it reprograms the cell’s machinery to produce viral proteins that replicate the virus’s genetic material.

These can be used to create new copies of the virus.
Influenza (the flu)
BacteriaThey enter the body and reproduce rapidly.

Many bacteria cause reactions in the host by releasing harmful toxins. These toxins damage tissues, which makes us sick.
ProtistsThey are eukaryotic organisms, which means they have a nucleus.

Some protists are very parasitic, so they live inside or on the surface of the host.
These protists reduce the amount of nutrients that the host can absorb.
FungiSome fungi reproduce through microscopic spores found in the air and soil.

These spores can land on you or they can be inhaled. Therefore, fungal infections usually start in the lungs or on the skin.
Illustrative diagram centred around a microscope, with four connecting branches leading to different types of microorganisms. To the top left is a pink box containing colourful representations of viruses. To the top right is a pale yellow box with images of bacteria. Further to the right is a beige box showcasing fungi with various mushroom-like shapes. Below the microscope is a pale pink box presenting protists, including a star-shaped and a worm-like figure. Each box is labelled with its respective microorganism category.

There are many ways that pathogens can spread from one organism to another. The spreading of pathogenic disease is known as transmission and the main types are listed in the table below:

Type of transmissionExamplesPrevention
Direct contact• Contact with bodily fluids

• Direct skin-to-skin contact

• Microorganisms from faeces

• Infected plant material left in the field
• Regular handwashing

• Practice safe sex

• Keep surfaces clean
VectorOrganisms that can transfer a pathogen from one host to another – Malaria is spread by mosquitoes.Killing vectors that spread diseases (e.g. mosquitoes)
WaterThe cholera bacterium can be found in water that is contaminated with faeces.Increasing access to clean drinking water
Unhygienic food preparationSalmonella food poisoning is typically caused by eating raw or undercooked meat.Prepare food safely
AirWhen an infected person sneezes, coughs or talks, they can spray thousands of tiny droplets containing virus particles, which can infect others.• Ventilate your home to keep fresh air circulating

• Cover your mouth and nose when you sneeze or cough

Once pathogens have established themselves in an organism, they can cause symptoms by releasing toxins or by damaging cells by invading and reproducing. It can take a while for any symptoms to appear, as a significant number of pathogens are needed for a host to start showing any signs of change.

The time taken between infection and the appearance of symptoms is called the incubation period. This can last for several hours to several years, depending on the type of pathogen. However, a human body can appear healthy for a long time after an infection because there may not be many pathogens present.