Why don't the bacteria in yogurt make us sick but other bacteria do?

Most are harmless: You have a common misunderstanding. There are millions of different types of bacteria, and 99.9% of them are harmless. Your body is filled and covered with trillions of bacteria, many of them required for a strong immune system and good health. In other words, harmless or helpful bacteria like those in yogurt are most common. Staph, strep, and others that cause illness are the rare exceptions.