Final answer:
A pathogen is a microorganism that causes infectious diseases. Types of pathogens include bacteria, viruses, fungi, and protozoa. Our immune system defends against these disease-causing agents.
Step-by-step explanation:
A pathogen is an agent, typically a microorganism, that causes disease in the organisms it invades. Infectious diseases are caused by such pathogens, which include bacteria, viruses, fungi, and protozoa.
These microorganisms can be transmitted in various ways, including through food, water, surfaces, and the air. Our immune system has evolved to protect us against these pathogens, using a complex array of specialized cells and molecules to fight off the disease-causing agents.