Pathogenic organisms are microorganisms that can cause disease in their hosts. These include bacteria, viruses, fungi, and parasites. When they invade the body, they can disrupt normal functions, leading to various health issues. Common examples of pathogenic organisms are Streptococcus pneumoniae, which can cause pneumonia, and the influenza virus, responsible for the flu. These organisms can spread through different routes, such as direct contact, contaminated food or water, and airborne transmission. Understanding how pathogenic organisms operate is crucial for developing treatments and preventive measures, such as vaccines and hygiene practices, to protect public health.