What is a pathogen and what role do they have in causing disease?

A pathogen is a harmful organism that causes infectious diseases. There are many types of pathogens: bacteria, fungi, viruses, protoctists, worms and insects. They allow diseases to be transmitted from person to person through a variety of methods including: normal social contact, though food and water, sexual contact or an animal bite. Pathogens can also be present in non-symptomatic people who are called carriers.