They invoke mysterious vibrations, resonance, force fields, or radiation totally unknown to science. The bark contains quinine, which helps in the treatment of malaria but also causes fevers.
Critics maintain that such minute doses are unlikely to have any significant effect on the body.
Homeopathy is very popular in Europe, especially among the Royal Family of Britain. It is also very popular in India, where there are more than 100 schools of homeopathy. No matter what ails you, treatment with a diluted like agent is the cure. Experience teaches otherwise. There seem to be countless examples one could come up which would contraindicate homeopathy as a reasonable approach to the treatment of disease.