How early can you take an infant out into the world on the regular?

Preventive care. I recommend that babies born at term and w/o complications be rather sheltered for the first 2 months after birth, especially in the winter season, where a common cold or flu could potentially cause significant respiratory distress in a young baby that would require hospitalization. If your baby is a former preemie, she needs special care at home and should be sheltered even longer than 2 months.
Safely at about 8 wk. Newborns are born with very little immune function except the antibodies passed from mom thru umbilical cord to baby. Gradually, newborns immune system begins to function and produce antibodies. At about 3 months old, the infant has also had vaccines and has now begun to produce antibodies to these life threatening diseases. Avoid persons who are ill and could cause fever in infant for 1st 3 mo.
Right Away. Minimize exposure to other people, because this is the primary source of infectious spread. Fresh air does not harbor too many infectious agents.Obviously there are weather extremes in temperature and moisture.But a bit of daily outside time is ok from day 1.