Bacteria are tiny, single-celled living organisms that are found almost everywhere on Earth, including inside and on the human body. They are microscopic and can have different shapes such as spheres, rods, or spirals. Bacteria are prokaryotes, meaning they lack a membrane-bound nucleus. While most bacteria are harmless or even beneficial—helping in digestion, nutrient absorption, and protecting against harmful microbes—some bacteria can cause diseases. They reproduce mainly through binary fission, dividing into two identical cells. Bacteria play key roles in ecosystems by recycling nutrients and are used in various industrial and medical applications. Humans have about as many bacterial cells as human cells in their bodies, often referred to as the microbiome, which is crucial to health.