Can a dog get a "stomach bug" from their humans?

You can go to this site for the complete list of conditions that can be passed between dogs & people. You would have to know exactly what type of stomach bug you had.

http://www.cdc.gov/ncidod/dpd/animals.htm

A complex question with a simple answer. Prepared dog food has only been in the last century. Dogs have food requirements as do people. If you use common sense many of the foods people eat are OK for animals. Some people used to cook (and still do) food at home for their pets. There are also pet vitamins. Dogs and cats as well can become diabetic with the wrong diet, ashmatic and many other illnesses. A dog in the wild would eat the WHOLE animal which would include most of the parts. My cats catch birds and do that now. (I can't control that). So if you intend to feed human food investigate what foods work and what don't on the net you will also find recipes. Hope that helps. Bad food in people and pets can make them grouchy. Slop, too much sugar, not enough protein, too much soft starch, not enough bulk etc.

Absolutely. Sheesh...if nothing else, amoebic dysentery could definitely be passed from human to dog.

Contaminated water is the substance you most likely share with your dog. Either the dog drinking from the toilet (after a human using it), or the well/tap water having problems.

No. However there are some infectious disease (Zoonosis) that humans can give to an animal and the other way around.

no, viruses are species specific

NO.