Pathogenic microorganisms are tiny living organisms that can cause diseases in humans, animals, and plants. These include bacteria, viruses, fungi, and parasites. When they enter a host, they can disrupt normal bodily functions, leading to various health issues. Common examples of pathogenic bacteria include Escherichia coli and Staphylococcus aureus. These microorganisms can spread through different routes, such as contaminated food, water, or direct contact with infected individuals. Understanding how these pathogens operate is crucial for developing effective treatments and preventive measures, such as vaccines and hygiene practices, to protect public health.