Secondary drowning occurs when someone inhales water. The fluid irritates and builds up in the lungs, making it difficult, if not impossible, for a person to breathe. Unlike typical drowning, the response is not immediate. In some cases, secondary drowning can occur 72 hours after the incident. In addition to filling the lungs with fluid, secondary drowning exposes the body to harsh chemicals if the fluid comes from a pool, hot tub, or other chemically enhanced body of water.
Source: Flickr user Brian Auer