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 stomach bug is highly contagious and most commonly occurs in the winter months from November and April. The most common way to catch the virus is from direct contact with someone who is sick. You can also catch the virus from contact with infected stool or vomit.