This section is from the book "A Compend Of The Principles Of Homoeopathy", by William Boericke. Also available from Amazon: A Compend of the Principles of Homoeopathy as Taught by Hahnemann.
Because here no bodily resistance has to be encountered, since a similar action is already existing in certain organs and tissues, the disease having overcome the resistance offered by the protective sphere of the body, and thus the affected region is exposed to attacks from without. The homoeopathic remedy acts upon the very tracts involved, and since the protecting gates are down, a much smaller dose is required. Another reason is the fact that the affected parts are probably rendered hypersensitive by the disease process. This is seen daily in every direction. Take a healthy eye, and it receives the rays of light and responds thereto; but let the same eye be inflamed, and see how exquisitely sensitive it becomes to otherwise natural stimuli. A third reason is that the homoeopathic remedy is given singly, without admixture with other drugs; and hence, its action is not interfered with.
Aggravation follows sometimes even a Minute Homoeopathic Dose. If so, it indicates that either the dose selected is still too strong, and in that case other symptoms will probably appear, or the remedy chosen was perfectly homoeopathic to the case and the aggravation would be but very temporary and would be in itself the best possible indication to do nothing further but wait.
Should aggravation follow a certain dose, it is a good rule to go higher in the scale of potency.