Shiitake Mushrooms are a must have during cold/flu season.  They may be a gentle assist with supporting the immune system.  As it may stimulate how well the immune system works, those with autoimmune issues should avoid using shiitsake mushrooms as it could increase the symptoms of the autoimmune disease.  
 
If you have hypereosinophilic syndrome, a blood disorder, don't use shiitake mushroom as it might make it worse. Speak to your physician or pharmacist before use if you are pregnant or thinking of becoming pregnant, nursing, or taking immunosupressant medications.