Bacteria are small living things that can be found almost everywhere. They live on the ground, in oceans, in the food that we eat and even in our bodies. They have been on earth long before there were any other organisms. Bacteria are so small you can only see them with the help of a microscope. They have only one cell and very simple structures. Bacteria have outer cell walls that protect the inside. A cell membrane is inside the cell wall. It prevents harmful substances from getting in and out. Inside the membrane is a soft, jelly-like substance called the cytoplasm. It has chemicals that help build the parts of a cell and break down food. Within it is the nucleoid, which contains the DNA. It controls a cell's growth and other activities Bacteria appear in different shapes. Spherical bacteria are round or ball-like. Rod-shaped bacteria live in your intestine and form chains. Spiral-shaped bacteria have a twisted form. Bacteria absorb nutrients and remove waste through their cell walls. They reproduce by splitting themselves in the middle. The two new cells can then split into four cells and so on. In a short time a single bacteria can reproduce billions of times. Some bacteria can get inside the human body, reproduce and cause an infection. These harmful bacteria are called pathogens. Our immune system is in charge of fighting off these bacteria. However, sometimes we need a vaccine to help fight off these intruders. Vaccines are weak or dead forms of the bacteria itself. Such a substance helps our body fight off bacteria in the future. Some examples of illnesses caused by bacterial infections are tuberculosis or pneumonia. The best way to protect yourself form bacteria is to keep yourself clean and wash your hands. Because bacteria may be transported by air, we should cover our nose and mouth when we sneeze. Vitamins also help strengthen our immune system and fight off bacteria. Bacteria can be helpful to the human body as well. They live in our stomach and help us digest food. Other bacteria create cheese and yoghurt. Bacteria are at work in the soil and decompose dead plants and animals.