epidemiology Epidemiology is the study and analysis of the distribution (who, when, and where), patterns and Risk factor (epidemiology), determinants of health and disease conditions in a defined population, and application of this knowledge to prevent dise ...

a susceptible individual (sometimes known simply as a susceptible) is a member of a

population Population is a set of humans or other organisms in a given region or area. Governments conduct a census to quantify the resident population size within a given jurisdiction. The term is also applied to non-human animals, microorganisms, and pl ...

who is at risk of becoming infected by a disease.

Susceptible individuals

Susceptibles have been exposed to neither the wild strain of the disease nor a

vaccination Vaccination is the administration of a vaccine to help the immune system develop immunity from a disease. Vaccines contain a microorganism or virus in a weakened, live or killed state, or proteins or toxins from the organism. In stimulating ...

against it, and thus have not developed immunity. Those individuals who have

antibodies An antibody (Ab) or immunoglobulin (Ig) is a large, Y-shaped protein belonging to the immunoglobulin superfamily which is used by the immune system to identify and neutralize antigens such as bacteria and viruses, including those that caus ...

against an

antigen In immunology, an antigen (Ag) is a molecule, moiety, foreign particulate matter, or an allergen, such as pollen, that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response. ...

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 the

placenta The placenta (: placentas or placentae) is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas, and waste exchange between ...

and from the

colostrum Colostrum (, of unknown origin) is the first form of milk produced by the mammary glands of humans and other mammals immediately following delivery of the newborn. Animal colostrum may be called beestings, the traditional word from Old English ...

, 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 as

malaria Malaria is a Mosquito-borne disease, mosquito-borne infectious disease that affects vertebrates and ''Anopheles'' mosquitoes. Human malaria causes Signs and symptoms, symptoms that typically include fever, Fatigue (medical), fatigue, vomitin ...

, these individuals make up such a small proportion of the total population that they can be ignored for the purposes of modelling an

epidemic An epidemic (from Greek ἐπί ''epi'' "upon or above" and δῆμος ''demos'' "people") is the rapid spread of disease to a large number of hosts in a given population within a short period of time. For example, in meningococcal infection ...

Mathematical model of susceptibility

The proportion of the population who are susceptible to a particular disease is denoted ''S''. Due to the problems mentioned above, it is difficult to know this

parameter A parameter (), generally, is any characteristic that can help in defining or classifying a particular system (meaning an event, project, object, situation, etc.). That is, a parameter is an element of a system that is useful, or critical, when ...

for a given population. However, in a population with a rectangular population distribution (such as that of a developed country), it may be estimated by: :

= \frac

Where ''A'' is the average age at which the disease is contracted and ''L'' is the life expectancy of the population. To understand the rationale behind this relation, think of ''A'' as the length/amount of time spent in the susceptible group (assuming an individual is susceptible before contracting the disease and immune afterwards) and ''L'' 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 estimating ''S'' 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 calculate ''S'' will be easily at hand. The parameter ''S'' is important in the mathematical modelling of epidemics.

Susceptibility in virology

Viruses are only able to cause disease or pathologies if they meet several criteria: #The virus is able to enter the cell (called a susceptible state) #There is a sufficient number of viruses within the cell #The virus is able to replicate within the cell (called a permissive state) 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 host

immune system The immune system is a network of biological systems that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to bacteria, as well as Tumor immunology, cancer cells, Parasitic worm, parasitic ...

, or abortive measures taken by intrinsic cell defenses.N.J. Dimmock et al. "Introduction to Modern Virology, 6th edition." Blackwell Publishing, 2007.

References

{{DEFAULTSORT:Susceptible Individual Epidemiology

Susceptible individuals

Mathematical model of susceptibility

Susceptibility in virology

See also

References