Best thing you can do for a cold or flu is to take the time to rest, and eat really nourishing food. Soup made using bone broth (boil bones of chickens or other animals) is fantastic, most spices (garlic, ginger, cayenne etc) are also great.
Other than that, I work with a lot of immune modulating/immune stimulating herbs. Most of them work best at the very first stages of immune infection (when you feel that first little scratch in your throat, or sense you might get sick). Some good ones are:
- Propolis: this it the stuff bees use to seal their hives, it is made from resins they gather from trees in their local environment. You usually find in tincture form. I take 20-30 drops any time I feel my throat is a little scratchy.
- Any of the medicinal mushrooms you mentioned (reishi, chaga, maitake, shiitake, turkey tail etc) are great. Medicinal mushrooms (especially reishi) are nice because they work at all stages of immune infection, not just the initial stage. So if you have a cold or flu that has moved deep into the body, they are probably your best bet
- Astragalus is a great immune supporting herb that specifically targets the lungs and spleen, so if an infection moves into the lungs it is a good option
- Steaming with essential oils daily can also help you recover a lot faster. Common ones like Eucalyptus, Tea Tree, Spruce or Fir work fine. Some oils will make you cough when you smell them, so are best used if you have a productive cough (one that is bringing up phlegm). If you have a dry unproductive cough (not bringing up mucus, but hurts), pick an oil with a licoricey scent (like Anise or Fennel), as these oils will calm the cough down. Sometimes if I have a nasal infection, I will snort a drop of Geranium (Pelargonium graveolens) oil into each nostril - it is a powerful antimicrobial.
- Licorice root tea is also good for soothing a dry cough
- Myrrh resin you can boil in water till it dissolves and drink it, has been shown to increase white blood cell counts significantly. You can also use frankincense or other tree resins, though not all of them will dissolve in water with boiling.
- If you have a high, spiking fever (and especially if you aren't sweating much, indicating a potentially dangerous fever), you can break it using a hot tea of yarrow (Achillea millefolium) which will induce sweating.
- Lots of people find gargling with salt water and/or using a neti pot helpful.
There's tons of other plants out there to play with... siberian ginseng, Echinacea, Usnea lichen, cat's claw, sagebrush, holy basil and so on. Try experimenting with some, and see what is available locally and works for you.
... oh yeah, it's kind of intense but for nasal congestion you can mix some ground horseradish root with apple cider vinegar and then squish it around in your mouth. Not for everyone, but it sure clears your sinuses out!!