Tracking dogs are usually bloodhounds or similar animals with acute senses of smell. Trackers can follow a trail through heavily wooded areas and even across streams or through busy apartment complexes. The scent comes from the dead cells that fall from a person's body and react with bacteria. The dog can smell these deteriorating dead cells both in the air and on the ground, even if the trail is almost a day old. They can also track in the rain, although heavy rainfall or strong winds hamper the tracking process. Tracking dogs also need human handlers to watch for physical evidence and interpret the dog's body language. Police officers considering the use of a tracking dog should avoid contaminating the trail. Police agencies purchasing tracking dogs should invest in dogs from reputable sellers and make sure that the dog is well-trained and has a skilled handler. Source of further information.