Sulphur
Every home needs some sulphur, it brings a little fire and energy to worn out systems.
For example, you have had a cold for a long time, and just can't seem to throw off those last little symptoms, the dry cough or tiredness. Sulphur fires up your immunity and gives it a boost, sufficient to take you back into your optimum health.
NB: Its also used for skin rashes but I would prefer that you contact me directly if you plan to use sulphur for this purpose. Homeopathic remedies can sometimes make things worse before they start to get better, and you don't want to do that without some expert guidance.