Our environment is full of bacteria. Our body is capable of having bacteria inside. http://www.scientificamerican.com/article.cfm?id=strange-but-true-humans-ca

Our environment is full of bacteria. Our body is capable of having bacteria inside. http://www.scientificamerican.com/article.cfm?id=strange-but-true-humans-carry-more-bacterial-cells-than-human-ones http://www.scientificamerican.com/article.cfm?id=strange-but... But when the immune system is not capable of handling them, you become sick.