Bacteria are ubiquitous microscopic living organisms that have only one cell. Millions (if not billions) of different types of bacteria can be found all over the world, including in our body. They are on your skin, airways, and mouth. They are also in your digestive system, reproductive system, and urinary tract. These single-celled living organisms may be beneficial. They make up your microbiome, which keeps your body healthy. Other bacteria can make you sick. Microbial contamination in spacecraft environments poses risks to both crew health and onboard systems. In-flight infectious disease caused by microbial pathogens does occur, and experiments indicate that spaceflight can increase the virulence. Bacteria represent a significant contamination challenge in spacecraft, and viruses were found to be present at relatively low levels.