Pathogenic bacteria are microorganisms that can cause disease in humans, animals, and plants. They invade the host's body, multiply, and produce toxins that disrupt normal bodily functions. Common examples include Escherichia coli, Staphylococcus aureus, and Streptococcus pneumoniae, which can lead to infections ranging from mild to severe. These bacteria can spread through various means, such as contaminated food, water, or direct contact with infected individuals. Preventive measures, like proper hygiene and vaccination, are essential to reduce the risk of infections caused by pathogenic bacteria. Understanding their behavior helps in developing effective treatments and public health strategies.