Worms.

Every dog is born with worms. It is very important that your dog is treated regularly against worms.
If your dog is infected with worms, its condition is probably not good. He will be a bit skinny and its coat won't be shiny. With a large worm infection, the worms will not only be in the intestinal tract, but they will start to spread throughout the body. They can finally come into vital life-organs, such as the heart and lungs and will cause many problems.
People can also be infected with worms through the dogs dirt; think about playgrounds for children where people take their dogs. My pups have already been wormed 3 times before they go to their new owners. I'll give the owner the advice to worm his dog every 3 months in the first year. After that once or twice a year should do, depending on where you live.
If you live in an area where there are a lot of dogs and you take your dog to a field where there is a lot of dog dirt, then there is a big chance of infection with worms. (The chance of infection is a lot smaller when you only take your dog out in your own garden). Make sure you give your dog the right worm-medicine, which works against all sorts of worms, and give the right dosage. Let your vet advice you!