There is lots of good advice on weston a price dot org (about lifetyle) and for diseases, I like earth clinic for ideas for home cures when bugs hit.

To me, every parent should read "Nutrition and Physcial Degeneration" by Weston Price, and/or Sally Fallon's Nourishing Traditions.

Hopefully the baby is breast fed and was a vaginal birth and hasn't yet been on antibiotics

If any of these things aren't true, they may want to right away think about populating that gut with good bacteria to create a strong immune system.

If lactation is no longer an option, there are recipes for home made formulas and I would find one, possibly made from raw goat or cow milk or if necessary, meat.