A wide range of organisms belonging to bacteria, viruses, fungi, protozoans, helminths, etc., could cause disease in man.

Such disease causing organisms are called pathogens.

$=$ Most parasites are therefore pathogens as they cause harm to the host by living in or on them. Pathogens can enter the body through various means, multiply and interfere with normal vital activities, which results in morphological and functional damage.

Pathogens must have the ability to adapt to life within the environment of the host.

For example, pathogens entering gut must know how to survive in stomach at low $\mathrm{pH}$ and resisting the various digestive enzymes.