Yes. Extensive decay can lead to serious infection. In the upper jaw that infection can get to your brain. In the lower jaw an infection can progress under your jaw closing off your throat. This is called ludwig's angina. Just recently an untreated boy died in baltimore.
Technically yes. An infection is an infection regardless of the cause. If the deep decay cause a major infection that spreads and compromises the rest of the body, death can result. Before antibiotics, tooth infections were one of the leading causes of death. That's why so many teeth were extracted at that time.