system
Given the following post, respond as if you're commenting on Reddit:
human
How does iodine purify water?
gpt
The mechanism by which halogens such as chlorine or iodine kill microbes is not fully understood. 

There are a few specific theories, of course, but I'll speak in general. 

In general, halogens are extremely unstable. They REALLY... REALLY want an extra electron. Because of this, it makes them extremely reactive, which means they react with... just about anything. Now, when this happens on a bacterial cell, it will often disrupt processes that allow the cell to survive.