# 15.1: New terminology

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- 25841

Terminology of the theory of disease is not completely consistent in the epidemiological literature, but we will use it consistently as follows.

*Virulence:* How much or how quickly a pathogen harms its host. Often symbolized as alpha, \(\alpha\), in disease equations. Example: if one-tenth of infected organisms die in a particular time period, and everything is random, \(\alpha\) = 1/10.

*Infectivity:* How readily a pathogen arrives at and invades a new host. Often symbolized as beta, \(\beta\), in disease equations. Example: in an otherwise uninfected population, if each infected host is expected to infect three others in a particular time period, \(\beta\) = 3.

*Basic reproductive number:* In an otherwise uninfected population, how many new infections an infected individual is expected to produce during the duration of the infection. Often symbolized as \(R_0\) in disease equations, and pronounced “are not.” This is a crucial number; if \(R_0\) is greater than 1, the disease will spread through the population, while if \(R_0\) is less than 1, the disease will die out.