Our nervous system can’t distinguish between being chased by a tiger and the nerves we feel just before a speech.” For urgent, life-threatening matters (which, unless you’re President Lincoln do not include public speaking), our bodies are doing their jobs by turning on stress hormones so we can run or fight for our lives.
It’s a glitch in our DNA to produce the fight/flight reaction because we are embarrassed by something, or anxious about some imagined future. Because our bodies are impressive in their ability to prioritize and dispatch resources, we weaken our immune system by starving it of resources and instead send them to non-urgent, non-emergencies.
Boost your immune system! Heal faster! Avoid getting sick to begin with! More practical reasons to do our best to proactively reduce stress. Not that we needed more reasons.