A relaxing, infused bath can super-charge immunity. How it works: Soaking in warm water spiked with apple cider vinegar raises the body’s pH to an optimal level, which makes the body a less hospitable environment for bacteria and viruses. I recommend doing the following twice a week during cold and flu season: Run a tub of the hottest water you can tolerate. Add 2 cups of organic apple cider vinegar. Soak until the water is cool, then don’t shower for at least 4 hours afterward to allow the vinegar to soak into your skin.