Since it was first discovered, we've learned that Legionella bacteria are common; they thrive in warm, wet environments such as hospital water pipes. They are so common, in fact, that most people come in contact with Legionella without any consequence. However, for people whose immune systems have been compromised, contact with Legionella can result in a deadly infection, typically as severe pneumonia. Because most pneumonias are not typed, most Legionella infections go unidentified and are reported simply as pneumonia of an unspecified cause.