A lot of people I know get really stressed out when their dog is sick but unless it is sickness because of malnutrition (not enough food) we need to remember that dogs, just like people, sometimes just get sick and you just need to give them some time to get over it. The best way to know if you need to be more concerned about it is to keep an eye on their poo. If it is discolored or has worms in it, it is time to go to the doctor, otherwise it is probably just something that they will get over with rest in a couple of days.
Dogs get illnesses just like humans do and they are actually a lot better at getting over it than humans are!