At 6-8 weeks of age puppies should receive their first vaccination; this is temporary and needs to be followed up with another one at 12 weeks. After the 12 week vaccination you can then take your puppy out in public areas.
You can be contagious from a few days up to two weeks or more, depending on which virus is causing your stomach flu (gastroenteritis).
Whether or not a person is contagious during the stomach flu incubation period will depend on which stomach flu virus the person is infected with. For example, with a rotavirus infection, the person is contagious during the stomach flu incubation period.