Susceptibility is the general capability of the organism to receive impression or the power of the organism to react to stimuli. It is one of the fundamental attributes of life. Upon it depends all the vital process, all functioning, physiological and pathological. Digestion, assimilation, nutrition, repair, secretion, excretion, metabolism ,catabolism as well as all disease process arising from infection and contagion depend upon the power of organism to react to stimuli. The cure and alleviation of symptom also depends upon the power of the organism to react to curative remedy.

Even though the term susceptibility is not directly referred in Organon Of Medicine, Kent quotes § 30,31,32,33 as bearing a likely explanation for the concept of susceptibility with regard to artificial morbific agents.

DEFINITION
H.A.Roberts- Defines susceptibility as the expression of vacuum in the individual. The vacuum attracts and pulls for the things most needed that are in the same plane of vibration as the want of the body.
 - Primarily susceptibility is the reaction of the body to external and internal influences.
S.Close- Defines susceptibility as the general quality or capability of the living organism to receive impressions and the power to react to stimuli.

Expression of susceptibility in vegetable kingdom, animal kingdom & in human
Vegetable kingdom: - Plants are influenced by circumstances and environment. They react to different circumstances and environmental changes in different ways. Some flora developing in certain altitude when swept by ocean to a different place may develop under an entirely different environment. The flora is able to react accordingly to the different stimuli. Here they have become immune to varying situation. But some may succumb to the adverse condition.

Animal kingdom: -Animals can withstand certain influences and hold their own under adverse conditions, which would be fatal to the another of the same species developed under differing circumstances. In other words some develop immunity under certain condition and may succumb to other adverse conditions.

Human kingdom: - One person will thrive in a vigorous climate where another will become seriously ill. One will thrive in dampness and other would succumb. Altitude influences some people kindly and others adversely.

Development of susceptibility
1) Heredity: - Susceptibility in a person is inherited from his parents. It depends on the miasmatic background of both the parents. It is also influenced by the mental condition and circumstances during the production of sperm and ovum. An interaction of all these determines the miasmatic background of the individual.
2) Early and late environmental factors: - may also influence the development of susceptibility. Early environmental factors involve condition pertaining to intra-uterine life. Any influence or indulgence during gestational period may affect the development of susceptibility of the individual. E.g. worries, diseases, tension, smoking, alcoholism, malnutrition etc.
Late environment factors also affect the development of susceptibility. These involve the influence during childhood and adulthood. e.g. fright during childhood, nutrition, any indulgences, smoking, tobacco chewing, alcoholism, worries and anxieties etc.

Thus both genetic and environmental factors determine the susceptibility of the individual.

Susceptibility in turn determines the quality of vital force.
I. Expression of Susceptibility
Susceptibility is expressed in physical, psychological and sexual levels. In areas of a) affinity b) attraction c) desires d) needs e) hunger.
These are expressed normally and are satisfied by the individual. But when they become morbid and perverted then disease, suffering and death occurs . They can also be altered by suitable modifying agents viz. drugs. These expressions of susceptibility can be taken as generals of the patient during case taking and thus the natural susceptibility of the patient could be found out.

II. Types of Susceptibility
a) Natural Susceptibility or normal Susceptibility: -.
According to H.A.Roberts -Reaction to stimuli under the healthy state of the individual could be recognized by the reaction of the individual to physical environment, food, remedies and its defense to toxic agents. Upon this normal susceptibility depends the health of the individual.
Natural susceptibility is the power of the organism to react defensively to a toxin, a contagion or an infection and to react constructively to food and drinks and curatively to the homoeopathic remedy.

b) Artificial Susceptibility: -
Kent speaks of artificial susceptibility in his Lecture of homoeopathic philosophy. When medicines are given they act for a time satisfying the susceptibility. They alter the susceptibility and when that dose of medicine is no longer able to act on altered susceptibility another dose or potency is required. This altered susceptibility that is arrived at is called artificial susceptibility.

c) Morbid Susceptibility: -
Susceptibility may become morbid and perverted under conditions of disease, suffering and death. According to S.Close morbid susceptibility could be regarded as a state of negative or minus condition-a state of lowered resistance.

d) Abnormal or Altered susceptibility: -
Abnormal susceptibility either demands excessive or defective stimuli or reacts to stimuli in an exaggerated or a diminished way.

Reaction- According to S. Close reaction is an expression of susceptibility .Reaction can be of two types 1) Excessive reaction 2) Deficient reaction.Excessive reaction or irritability is a condition sometimes met where the patient seems to suffer an aggravation from every remedy without corresponding improvement. This is a state of general hypersensitiveness.

In Idiosyncrasy: - It is a constitutional reaction, which makes a peculiar constitution predisposed to a peculiar agent even in a very much minute quantity at all the times.

111.Deficient reaction - Deficient reaction may occur in cases where improvement is found to cease and well selected remedies fail to act .
Under such circumstances it is necessary to give some intercurrent remedies .

e) Morbid susceptibility implies negative or diminished condition. (S.C)
Diminished susceptibility: - Susceptibility can be diminished by forcibly using heterogeneous medication, antitoxins, antiseptics, external applications, vaccination etc. Uses of stimulants and depressants like alcohol, brandy etc and inoculation of crude pathological products like animal sera and vaccines may leads to diminision of susceptibility .

IV. Destruction of susceptibility: - Total destruction of susceptibility means total destruction of reactivity of the body or death.

Partial destruction of susceptibility means serious impairment that makes the patient chronically invalid and impossible to cure. Comma and suspended animation occur due to the partial destruction of susceptibility.

V. Demands of Susceptibility
Susceptibility according to H.A.Robert is an expression of vacuum in the individual. This vacuum attracts and pulls the things most needed that are in the same plane of vibrations as the want of the body.

Natural susceptibility: - Demands of natural susceptibility include the demands in the normal healthy state in the areas of hunger, affinity, attraction, desire and need. E.g. hunger demands food.
Altered susceptibility: - Demands either excess or deficient stimuli to satisfy it. E.g. loss of appetite, abnormal hunger, excessive craving for special foods.

Morbid susceptibility: - When susceptibility becomes morbid and perverted it demands external morbific agents in other words there is an influx of disease causing agents resulting in disease.

If the morbid susceptibility demands symptomatically similar medicine or a similimum it results in cure. If it is satisfied only by a partial similimum it results in palliation.

If a defective similimum i.e. nonhomoeopathic medication or antiseptics, antitoxins, external applications etc are used it leads to suppression or sometimes a zigzag cure.

VI. Factors which modify susceptibility
1) Age: - Susceptibility is greatest in young vigorous persons and children and it diminishes with age. Children are particularly sensitive during their developmental period .The most sensitive organs are those, which are being developed.
2) Constitution and temperament: - High potencies are adapted to nervous , sanguine and choleric temperament and also to intelligent, intellectual ,zealous and impulsive persons who are quick to act and react. Lower potencies and more frequent dose correspond to torpid phlegmatic individuals, coarse fibered, sluggish individuals of gross habits and to those of great muscular power who require powerful stimuli to excite them.
3) Habit and environment: - Susceptibility is increased by intellectual occupation, by excitement of imagination and emotions, by sedentary occupation by long sleep and effeminate life. Therefore high potencies are required. A person who is accustomed to long and severe labor out of doors who sleep little and whose food is coarse is less susceptible. Persons expose to continuous influence of drugs such as tobacco worker and dealers, distillers and brewers and all connected with liquors and tobacco trade, druggist, perfumers, chemical workers are less susceptible and require low potencies. An idiot, imbeciles, deaf and dumb has low susceptibility hence require low potencies. But persons who have taken many crude drugs of allopathic, homoeopathic or bargain counter prescription often require high potencies for their cure. Here high potency is effective because it acts on virgin soil and invades new territories.
4) Pathological conditions: - In certain terminal conditions the power of organism to react even to indicated homoeopathic remedy is low therefore material doses are required. This may be due to existence and gross pathological lesions, long existent exhausting chronic disease or much previous treatment. If the grade of disease is low, power of reaction is low; the remedy must be given low.
5) Seat character and intensity of the disease: - In certain malignant rapidly fatal diseases like cholera susceptibility is low so it requires material doses or low potencies. Diseases characterized by diminished vital action , torpor, collapse require lower potencies and increased vital action requires high potencies.
6) Previous abuse of medicine: - Due to this we may find that the patient is not at all sensitive even to the indicated remedy. Then all medication has to be ceased for few days. Then carefully regulate the diet and regimen. Hahnemann recommends the administration of opium in one of the lowest potencies every 8th or 12th hour until some signs of reaction are perceptible. By this means, susceptibility is increased and new symptoms of disease are brought to light. Carboveg, Sulphur and Thuja are other remedies, which serve to arouse the organism to reaction so that indicated remedies will act.
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VII. Clinical assessment of Susceptibility
Clinical assessment of susceptibility helps us in determining the potency, dose and also the repetition of medicine. While assessing susceptibility one should bear in mind its modifying factors like age, habits, environment, pathological conditions, Idiosyncrasy, homoepathicity etc.

Susceptibility can also be assessed Jahr’s law.
JAHR’S LAW: - The more similar the remedy, the more clearly and positively the symptoms of the patient takes on the peculiar and characteristic form of the remedy, the greater is the susceptibility to that remedy and higher is the potency required.

The practical bearing of this on the selection and potency or dose is that-in a given case where the symptoms are not clearly developed and there is absence or scarcity of characteristic features or where 2 or 3 remedies seem about equally indicated, susceptibility and reaction may be regarded as low. Therefore we give remedy, which seems most similar in a low potency. But when the symptoms of a case clearly indicate one remedy, whose characteristic symptoms correspond closely to the characteristic symptoms of the case we give high potencies.

Stuart Close modifies Jahr’s advice by suggesting that the clearer and more positively the finer , more peculiar and more characteristic symptoms of the remedy appear in a case, the higher the degree of susceptibility and the higher the potency required .

Illustration of Jahr’s concept
Low potencies
ARS
Radii lie close to each- RHUS other and Ars, Rhus, Bry,
Sulph has many symptoms- BRY SULPH in common.
High potencies RHUS
Radii recede from each other. ARS  Ars, Bry, Rhus, Sulph appear more
And more distinctly in its peculiar
Characteristic features. BRY SULPH

VIII. Utility of Susceptibility
Knowledge of susceptibility helps in assessing the attitudes and reactions of a living being to varying environmental conditions.
In the healthy normal state, the personality and behavioral traits could be understood by knowing his affinities, attractions, desires and needs. This helps in making and maintaining interpersonal relationships. Bearing of hardships and its effects, the effects of joy , pleasant surprises , good news and success upon the individual, his attitude and reaction towards adversities etc could be understood. This will help us in individualizing the person, helps us to understand his constitutional make up ,his temperament and his miasmatic background.
In a diseased person, the knowledge of susceptibility helps us in satisfying the morbid susceptibility by giving a similimum. Living organism is much more susceptible to homogenous or similar stimuli. Selection of similimum depends on the susceptibility. For similimum corresponds to the more peculiar and characteristic feature of the drug therefore makes the individual highly susceptible to the drug. Similarly in selection of dose and potency the knowledge of the kind and degree of susceptibility is important. Highly susceptible individuals require high potencies and low doses where as less susceptible require low potencies and massive doses. Prognosis of a case is also depended upon the susceptibility of the individual. Power of the person to react to the medicinal stimuli makes a good prognosis where as a lack of reaction leads to a poor prognosis.

IX. Duty of the physician
(S.C) The ability to modify susceptibility is the basis of the art of the physician. If the physician knows how to modify susceptibility in such a way as to satisfy the requirements of the sick organism and bring about a true cure, then he is a physician indeed. For cure consists of simply satisfying the morbid susceptibility of the organism.
Physician should know how to maintain normal susceptibility and he should also know the factors, which hinder the natural susceptibility.

Physician should never react to any methods that will diminish or destroy the susceptibility. During the time of Hahnemann, old school adopted many torturesome method which did nothing to advance the process of cure but contributed to the lowering of resistance or susceptibility. Such methods as the specifics, derivatives, repellent, medication, counter irritants, stimulating and strengthening agents, mixture prescriptions etc against which Hahnemann fought with an undaunted energy.

At present the modern school resorts to the inoculation of crude pathological products of animal sera and vaccines, antitoxins, antiseptics etc which also lead to diminision of susceptibility. The action of endotoxins have led to status infectiosus- a sterile death were the bacteria have been fully destroyed but there is an active degeneration of the organs.

Stimulants and depressant like alcohol, brandy and strychnine also leads to destruction or decrease of susceptibility.

The physician should be able to correctly assess the patient’s susceptibility. He must not waste his time by violent measures. The imaginary idea of violence, the malignity and rapidity of the disease should not lead the physician to believe that it should be treated with violent heroic medicines. This would be like practicing homoeopathy with a vengeance. (s.c)When susceptibility is lowered, when everything indicates that life and strength are at lowest ebb and store of vital energy is almost exhausted, physician should not use any violence but should treat in a mildest and most delicate means. Proper use of stimulants is advisable. But physiological drug stimuli should be legitimately filled only in exceedingly small or rare instances.

Reference books:
1) Lectures and essays on homoeopathic philosophy – S.Close
2) Lectures on homoeopathic philosophy - J.T.Kent
3) The principles and art of cure by homoeopathy – H.A.Roberts