Do you mean hydrotherapy?
Hydrotherapy is used for many things, and is contraindicated for certain types of wounds, etc.
Generally, it involves a pool of water heated to a specific temperature. It can either be hot or cold depending on the needs of the client.
Heat increases the inflammatory response by increasing the metabolic activity in the area.
Cold can slow down inflammation, cause numbness, etc., which can allow the client to perform certain activities for up to 20 minutes after the therapy is administered.
When talking about infection, it depends on the type of hydrotherapy. There are cases where you should not use hydrotherapy because it acts to "slough off" layers of a wound to facilitate healing.
I tried to keep this as simple as possible! Hope this answers your question!