Baby’s 1st time getting sick

I have a 2-week-old and I am just so worried about her getting sick now that cold and flu season will soon be here in full force! I have crazy new mom anxiety about this and have been taking her temperature with a temporal scanner probably way more than I need to. She is breastfeeding and I know she’s building an immune system from that but I still can’t help but be worried. We have already had several visitors (we always make them wash or sanitize their hands) and I’m wondering if I should put a stricter limit on visitors. How old was your baby the first time he or she got sick? How old was your baby when you started to let visitors hold and interact with him/her?