Pathogens include bacteria, viruses and fungi. Disease can result from pathogenic microorganisms penetrating any of an organism’s interfaces with the environment. These interfaces include the digestive and gas-exchange systems. Pathogens cause disease by damaging the cells of the host and by producing toxins.

Micro-organisms are ones so small they cannot be seen without a microscope. Harmful micro-organisms are called pathogens and bacteria, fungi and viruses are all examples.

These organisms can infect the body by penetrating it interfaces with the environment- this means entering the body by a surface that links the inside with the outside, for example the digestive system and the gas exchange system.

When inside the body pathogens causes disease in two ways:

• They damage cells of the host (body) so they can't function properly
• They produce toxins which can alter the workings of the infected organism.