Pathogens are microorganisms that cause disease in their hosts, which can be animals, plants, or humans. Common types of pathogens include bacteria, viruses, fungi, and parasites. They can infect hosts through various routes, such as ingestion, inhalation, or direct contact. Understanding pathogens is crucial for developing treatments, vaccines, and preventative measures to combat diseases. Additionally, the study of pathogens contributes to fields like microbiology, immunology, and public health.