Final answer:
A pathogen is a microorganism that causes disease by invading and harming its host. There are primary and opportunistic pathogens, with the former able to cause disease in healthy individuals and the latter in those with compromised immune systems. The human immune system has evolved to protect us against these disease-causing agents.
Step-by-step explanation:
A pathogen is an agent, typically a microorganism such as bacteria, viruses, fungi, or protists, that causes disease in the organisms that it invades. Pathogens are often referred to as germs and are responsible for a wide array of infectious diseases. These organisms can transmit diseases and can vary significantly in their virulence, the ability to cause disease.
Pathogens can be categorized into two main types: primary pathogens, which can cause disease in healthy individuals, and opportunistic pathogens, which usually cause diseases in situations where the host's defenses are compromised, such as in individuals with a weakened immune system. Pathogens cause disease by invading host tissues, multiplying, and often releasing toxins that damage host cells.
In response to pathogenic threats, the human immune system has evolved to protect the body using specialized cells and molecules. It defends against a majority of pathogens present in the environment, which we encounter through food, water, surfaces, and air.