In the UK the advice is that it is safe for pups to go anywhere two weeks after the second puppy shot - the first is usually at 8 weeks, and the second at 10 weeks. Until then precautions are recommended - which doesn't mean not taking the puppy out, which would play havoc with essential socialisation, etc, but does mean avoiding high risk areas and high risk dogs. Most puppy classes check vaccination status and disinfect the premises appropriately, and friends' houses with vaccinated dogs should be safe. These days in developed countries more pups suffer because of inadequate socialisation in the crucial early weeks than because of infectious diseases - if you miss that window of time when their response to new stuff is to welcome it and explore it you are likely to be playing catch up for a long time to come.
To sit with a dog on a hillside on a glorious afternoon is to be back in Eden,
where doing nothing was not boring- it was peace.
~ Milan Kundera