A pathogen is any organism that causes disease.
These can include microorganisms and larger organisms, such as tapeworms.
Pathogenic microorganisms include bacteria, viruses and fungi.
Pathogens cause disease by:
1) Producing toxins - many bacteria release toxins (harmful molecules) into the body.
2) Cell Damage - Pathogens can physically damage the host cells by:
- Rupturing them to release nutrients (proteins etc.) inside them.
- Breaking down nutrients inside the cell for their own use. This starves and eventually kills the cell.
- Replicating inside the cells and bursting them when they're released e.g. some viruses do this.
Pathogens need to enter the body to cause disease.
They get in through an organism's suface of contact (interface with the environment). An organism has 3 main interfaces with the environment:
Gas Exchange System - If you breathe in air that contains pathogens, most of them will be trapped in mucus lining the lung epithelium. These cells also have cilia (hair-like structures) that beat and move the mucus up the trachea to the mouth, where it's removed. Unfortunately, some pathogens are still able to reach the alveoli where they can invade cells and cause damage.
Skin - If you damage your skin, pathogens on the surface can enter your bloodstream. The blood clots at the area of damage to prevent pathogens from entering, but some may get in before the clot forms.
Digestive System - If you eat or drink food that contains pathogens, most of them will be killed by the acidic conditions of the stomach. However, some may survive and pass into the intestines where they an invade cells of the gut wall and cause disease.