©
Shutterstock
Bacteria are tiny single cell organisms, invisible to the naked eye, that live in essentially every possible habitat on our planet. Plants and animals are covered with microorganisms, the soil and the oceans are teaming with bacteria, and it is estimated that bacterial cells actually outnumber human cells in the body by a factor of 10-100:1. The overwhelming majority of bacteria are completely harmless, but a small minority of pathogenic bacteria can cause infections in humans. For most of human history, bacterial pathogens have been a major cause of disease and mortality. For example, the plagues that ravaged Europe in the middle ages were caused by the bacterium Yersinia pestis, and tuberculosis, and cholera outbreaks are caused by the bacterium Vibrio cholera.
Read more (Oxford Science Blog, University of Oxford)