Gut bacteria, which number more than 100 trillion cells, have more of an impact on our health than medical experts previously realized. But not all gut bacteria are created equal. Among other things, “good” gut bacteria improve digestion, strengthen the immune system, and manufacture the vitamins your body needs. On the other hand, “bad” gut bacteria can cause digestion nightmares, mental problems, skin conditions, and all kinds of other problems you’ll learn about in just a second.