A pathogen is a microorganism, such as bacteria or viruses, that can cause disease in its host. The term refers to any agent that produces an infectious disease within a living organism. Pathogens are responsible for causing illnesses ranging from mild infections to severe and life-threatening diseases. They can be transmitted through various means, including direct contact with the infected individual or contaminated surfaces, ingestion of contaminated food or water, or through vectors such as mosquitoes that transmit pathogens from one host to another.