In this episode, we learn about the war going on inside our bodies every day. We generally think of our immune systems as defending us from malicious, foreign attackers. But, as always with biology, we’re finding that it’s not that simple. In some cases, an apparent foe might turn out to be a friend, and vice versa. Here we bring you three different stories about how the immune system can be outsmarted, misdirected, and even re-engineered.