I think that was probably good advice actually. Blood in stools CAN be very serious BUT is is also common in dogs. Mostly it clears up in a day or two on its own but generally they do suggest stopping all food for 24 hours or so then feeding something bland for the next few days. The thing is if it gets really bad and becomes a serious level of bleeding (haemmorhagic) gastroenteritis, it can become severe and even fatal. Mainly as they dehydrate severely and don't get enough water when passing the bloody stools.
But mine get this on and off -- Lily is prone to it -- and at this point I do not usually take them to the vet because 1) I know the difference between it being mild and severe; 2) it can generally be managed in the way your vet suggested; 3) I know what works for my gang. If any of them has it severely and is distressed of course they go to the vet.
Yes they can pass it amongst themselves -- if one gets it the others generally do. It can be caused by many things including eating stuff off the streets, getting into trash, worms, bacteria, raw diets, etc. I have had a bout across them all after eating raw beef recently.
I find withholding food for 24 hours then giving very small amounts of James Wellbeloved tends to settle their tums and this is really hastened by giving those probiotic pastes like Prokolin or... hmmm there's another I can't think of. It is really very much worth buying this from one of the vet sites in the uk where the cost is nearly half what Irish vets charge for it. I find a day of those probiotic pastes and their tums settle completely -- problem is if they don't get it long enough (say 4-5 days) the gastro problems can return. I now keep this on hand mainly for Lily.
I recently had all 5 getting colitis in rounds over about three weeks -- do not know what they picked up but it would not go away! Started when we were in Donegal for a week. No sooner was one better than another had it. But eventually it cleared.
You can also give about 1/4th a tablet of immodium depending on a dog;s size -- this tends to help calm things quickly -- here is advice on what to give (watch for the contraindications!):
http://www.peteducation.com/article.cfm?c=0+1303+1459&aid=1432
Generally if a dog gains weight the solution is the same as people -- cut down on the amount fed, make sure other family members are not sneaking treats (dental treats are VERY high in calories for example as is cheese, pigs ears etc), that the dog isn't getting calories from sneaking cat poops in the litter box or garden, and give more exercise! Even a light food can cause a dog to gain weight. Cut meals by a third; if the dog doesn't lose anything over the next coupe of weeks cut by half. I think many people feed way too much -- three of mine for example get less than 1/2 cup of food per day in their actual meals.
In rare cases weight gain may be a thyroid problem.
I find James Wellbeloved to be a great food -- all the dogs are very regular on it, never upsets any tummies, and they like all the flavours.