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Inepidemiology asusceptible individual (sometimes known simply as a susceptible) is a member of apopulation who is at risk of becoming infected by a disease.
Susceptibles have been exposed to neither the wild strain of the disease nor avaccination against it, and thus have not developedimmunity. Those individuals who haveantibodies against anantigen associated with a particular infectious disease will not be susceptible, even if they did not produce the antibody themselves (for example, infants younger than six months who still have maternal antibodies passed through theplacenta and from thecolostrum, and adults who have had a recent injection of antibodies). However, these individuals soon return to the susceptible state as the antibodies are broken down.
Some individuals may have a natural resistance to a particular infectious disease. However, except in some special cases such asmalaria, these individuals make up such a small proportion of the total population that they can be ignored for the purposes ofmodelling anepidemic.
The proportion of the population who are susceptible to a particular disease is denotedS. Due to the problems mentioned above, it is difficult to know thisparameter for a given population. However, in a population with arectangular population distribution (such as that of a developed country), it may be estimated by:
WhereA is the average age at which the disease is contracted andL is the life expectancy of the population. To understand the rationale behind this relation, think ofA as the length/amount of time spent in the susceptible group (assuming an individual is susceptible before contracting the disease and immune afterwards) andL as the total length of time spent in the population. It thus follows that the proportion of time spent as a susceptible is A/L and, in a population with a rectangular distribution, the proportion of an individual's life spent in one group is representative of the proportion of the population in that group.
The advantage of estimatingS in this way is that both the average age of infection and life expectancy will be well documented, and thus the other parameters needed to calculateS will be easily at hand.
The parameterS is important in themathematical modelling of epidemics.
Viruses are only able to cause disease or pathologies if they meet several criteria:
Hence susceptibility only refers to the fact that the virus is able to get into the cell, via having the proper receptor(s), and as a result, despite the fact that a host may be susceptible, the virus may still not be able to cause any pathologies within the host. Reasons for this are varied and may include suppression by the hostimmune system, or abortive measures taken by intrinsic cell defenses.[1]
