Echinacea is in the daisy family, so for those that may be allergic or have air-borne allergies, ask your doctor for other alternatives. As always, seek the advice of your physician before taking any medications, herbal alternatives, changing your eating patterns or exercise regimen.
Echinacea is a popular herbal remedy for strengthening the immune system. I take a double dose immediately upon feeling "not quite myself" or "I think I'm coming down with something". Then I take a single dose for 3 to 4 days as needed. I have only had a flu or cold twice in the last 12 years and both times I had run out of Echinacea and didn't think to get some. It tends to work much better for colds (for me), but it does decrease the severity and length of the flu (as compared to those around me who 'shared' the virus).
Keep in mind that any immune-building herb or product can be detrimental if overused. Continual daily use or long-term use has been known to weak the immune system, so it is best to only use this when you need it.
If you are unable to use Echinacea, ask your doctor about Siberian ginseng, wild indigo, thuja and chamomile. These other herbal alternatives also boost the immune system and help in fighting disease.