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).
The contagious period for stomach flu varies depending on the virus that is causing your symptoms. Stomach flu is not really influenza but viral gastroenteritis, an illness characterized by vomiting and diarrhea. Viral gastroenteritis is caused by several different highly contagious viruses. Among the most common are noroviruses and rotaviruses.