Bacteria are single-celled microorganisms that exist in various environments, from soil to the human body. They play crucial roles in processes like decomposition and nutrient cycling, and some bacteria are essential for human health, aiding in digestion and producing vitamins. However, not all bacteria are beneficial; some can cause diseases in humans, animals, and plants. Pathogenic bacteria, such as Escherichia coli and Streptococcus pneumoniae, can lead to serious health issues. Understanding bacteria is vital for medicine, agriculture, and environmental science.