epidemiology Epidemiology is the study and analysis of the distribution (who, when, and where), patterns and determinants of health and disease conditions in a defined population. It is a cornerstone of public health, and shapes policy decisions and evide ...

, force of infection (denoted

\lambda

) is the rate at which

susceptible Susceptibility may refer to: Physics and engineering In physics the susceptibility is a quantification for the change of an extensive property under variation of an intensive property. The word may refer to: * In physics, the susceptibility of ...

individuals acquire an

infectious disease An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable di ...

. Because it takes account of susceptibility it can be used to compare the rate of transmission between different

groups A group is a number of persons or things that are located, gathered, or classed together. Groups of people * Cultural group, a group whose members share the same cultural identity * Ethnic group, a group whose members share the same ethnic ide ...

of the

population Population typically refers to the number of people in a single area, whether it be a city or town, region, country, continent, or the world. Governments typically quantify the size of the resident population within their jurisdiction using a ...

for the same infectious disease, or even between different infectious diseases. That is to say,

\lambda

is directly proportional to

\beta

; the effective transmission rate. :

\lambda = \frac

Such a calculation is difficult because not all new infections are reported, and it is often difficult to know how many susceptibles were exposed. However,

\lambda

can be calculated for an infectious disease in an

endemic Endemism is the state of a species being found in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also found else ...

state if homogeneous mixing of the population and a rectangular population distribution (such as that generally found in developed countries), rather than a

pyramid A pyramid (from el, πυραμίς ') is a structure whose outer surfaces are triangular and converge to a single step at the top, making the shape roughly a pyramid in the geometric sense. The base of a pyramid can be trilateral, quadrilat ...

, is assumed. In this case,

\lambda

is given by: :

\lambda = \frac

where

A

is the average age of infection. In other words,

A

is the average time spent in the susceptible group before becoming infected. The rate of becoming infected (

\lambda

) is therefore

1/A

(since rate is 1/time). The advantage of this method of calculating

\lambda

is that data on the average age of infection is very easily obtainable, even if not all cases of the disease are reported.

References

{{Reflist

See also

References

Further reading