Grapes are toxic to dogs and not humans because the grapes contain a compound that is poison to a dogs kidneys.
The compound that causes the kidney failure in dogs that is found in grapes is not fully known as of now.
But if a dog eats a large amount of grapes then they can get kidney disease and then die as a result of the kidney failure from the grapes.
If your dog ate just one or a couple grapes then they are unlikely to die from it but eventually the dog could die especially if they eat a bowl of grapes.
My dog has eaten a couple of grapes and I called the Vet and the vet said that the dog should be just fine.
He said it was the kidney failure that kills the dog and ti would not happen right away.
But my dog is still living and it has been 5 years since he got into the grapes.
That compound found in the grapes has no bad affect on human kidneys but for some reason it does on dogs kidneys.