When people refer to being “as sick as a dog”, it is in reference to dogs never seeming to get a little sick, but getting severely sick. Dogs are very resilient animals. They can digest things that are not food (plastics, metals, etc.) and not get ill. They can withstand extreme weather and temperatures and bypass getting sick. But when a dog does become ill, it is usually severe, and always needs the attention of a veterinarian.

This would explain why people started using “as sick as a dog” to describe how sick they felt by referring to the canine’s degree of severe sickness.

Everyone seems to secretly understand that being “as sick as a dog’ automatically means that person is severely sick, even if they know hardly anything about dogs. Feel free to head on over to this website of the vets Grandview to learn more.