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 Incubation Period of Stomach Flu. To understand the differences in incubation periods for stomach flu, know that there are two viruses that cause stomach flu: norovirus and rotavirus. Norovirus Norovirus is the most common cause of stomach flu. Most people are contagious from the moment that they become ill.