Echinacea is an immune-stimulant, which means it stimulates your immune system into activity. The best course of action is to take it as soon as you see the first sign of a sniffle, sore throat or cough, so that your immune system has the extra boost that it needs to tackle the infection before it really takes root. If it can start activating your defense system before things really get rolling, it can shorten how long you are sick for and get you back to your daily activities faster.
 
There is no evidence that your body builds up any resistance to echinacea. If you are concerned about compromised immunity over the long-term, take a standardized echinacea supplement daily for a few weeks then take a week “off” and then repeat the cycle. This can help protect you or shorten the duration of a cold or flu if you do get sick.