"Pathogenic" refers to organisms, such as bacteria, viruses, fungi, or parasites, that can cause disease in a host. These pathogens invade the body, multiply, and disrupt normal biological functions, leading to illness. Understanding pathogenic organisms is crucial for developing treatments and preventive measures. In medicine, identifying pathogenic agents helps in diagnosing infections and determining appropriate therapies. For example, Streptococcus pneumoniae is a pathogenic bacterium responsible for pneumonia, while HIV is a virus that leads to acquired immunodeficiency syndrome (AIDS). Recognizing these pathogens aids in public health efforts to control and prevent disease outbreaks.