Often, I see herbs misapplied during illness due to misinformation spread in popular blogs and market trends. Once an herb or preparation becomes trendy, it is misused and misapplied at an alarming rate. Or, one simple hears of an herb that is "good for" something, and uses it without understanding its actions. One of the most common mistakes I see is when one uses herbs for the immune system, and doesn't know which herbs or group of Herbs is most appropriate for their current health state.
There are different categories of Herbs that interact with the immune system, most commonly known as Immunostimulants, Immunomodulators, and Adaptogens. Other groups - which are not necessarily exclusive, Herbs can have actions that can put them in multiple categories - include Antimicrobial and Antiviral. (note, Herbs have all kinds of active constituents and work on multiple organs and organ systems within the body).
So, what is the difference? I'll be very brief here, and simply list a couple examples for each group, as well as a very general overview of what they do.
Immunostimulants - these stimulate the immune system. They are taken short term, usually in frequent doses, and meant for acute conditions - not chronic - and are not meant to be taken during illness, when your immune system is already stimulated by pathogens, but at the start of illness. The most popular example here is Echinacea, which is also an overharvested herb.
Immunomodulators - these don't stimulate the immune system, but rather help balance it. They boost immunity for those who experience frequent illness, and help balance the immune response. These can be taken long term and have a deeper effect on the body than immunostimulants. A lot of mushrooms, like reishi and cordyceps are considered Immunomodulators. Other herbs like Elder are also considered a part of this group, and Elderberry Syrup is likely the most popular example you'll see for winter time wellness preparations in this group.
Adaptogens seem to be grouped in the Immunomodulating category much of the time, but are substances that help the body adapt to stress. I usually see people recommending these for illness when in reality they ought to consider a nervine or a lifestyle change, as many people want to throw herbs at a problem to cope instead of resting and letting their body focus on fighting infection.
Antimicrobial/Antiviral - pretty much as they sound, they act against bacteria or viruses to prevent or eradicate infection. They can be used topically in salve or tincture form as first aid, or taken internally as food or medicinal forms like tinctures. Many culinary spices and herbs like garlic and ginger are in this group, and are popular in preparations like Fire Cider. It's important to understand which microbes these herbs act against, as some are best suited for certain infections, not all will act against a certain microbe to the same effect.
Herbs are complex beings, with multiple actions on the body across various organs and systems. There are centuries of traditional use, and some modern research, that help guide us as we incorporate them into our daily diets and medicine cabinets. Categorizing an herb does not limit its abilities, and there are various herbal traditions and medicine systems that apply the same herbs in different ways.
However, it is imperative to really research an herb before using it, to understand its various constituents and actions before consuming it or applying it for an illness or condition, to find the medicine best suited to your needs. Also, it is important to note that often, illness is a sign of imbalance, a call for us to slow down, get adequate rest, improve our diet, and make some healthy changes to be at our best. And to treat holistically, we need to wisdom to heed our body's signs, and make the necessary adjustments - simply loading up on herbs without doing anything else will do little for you.
If you'd like to see a nice, in depth article about herbs and the immune system, I highly recommend Chestnut School of Herbal Medicine's post written by Juliet Blankespoor, which you can find on their blog.