system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How dogs are able to sense and know when something medically wrong is about to happen in a person before it actually happens?
gpt
This depends on the effect you mean.    
   
Some cancers can be smelled by dogs. There are 'cancer sniffing dogs' that have been trained to detect cancer and are better and cheaper than our imaging equipment.    
   
So they're using their superior senses to pick up on things that humans can't detect.