Bacteria are single-celled microorganisms that can be found in various environments, including soil, water, and even inside the human body. They can be beneficial, such as those that help with digestion, or harmful, causing diseases like strep throat and tuberculosis. Bacteria reproduce by dividing, which allows them to multiply quickly. Viruses, on the other hand, are much smaller than bacteria and cannot reproduce on their own. They need a host cell to replicate, often causing illness in the process. Common viral infections include the common cold and influenza. Unlike bacteria, viruses are not considered living organisms because they lack cellular structure.