There’s also no single cause for a swollen battery. Some are due to manufacturing defects, others are caused by the age of the battery, and still other cases can be caused by misuse, such as not properly exercising the battery over time, or by using the wrong power charger.
first step is to exercise caution. Puncturing a battery in any state is incredibly dangerous, but swollen batteries are especially vulnerable to compromise as their casing is already under stress from the built up gasses within. In short, handle any device with a suspected swollen battery with care.
In the meantime to protect yourself I wouldn't try plugging it in or leaving it plugged in because that may further stress the battery. Hopefully if you have a backup stored to icloud or iTunes you can recover your contacts, mail etc from the backup. If you would like to dispose of the phone I would highly reccomend taking it to the apple store and have them dispose of it.