La tasa de letalidad es la proporción de personas que mueren por una enfermedad entre los afectados por la misma en un periodo y área determinados. Es un indicador de la virulencia o de la gravedad de una enfermedad. Es difícil de realizar porque es preciso registrar todos los enfermos por esa enfermedad y en muchas enfermedades los casos asintomáticos o leves no son declarados. Fórmula: \n* L: tasa de letalidad. \n* F: Número de muertes por una enfermedad en un periodo y área determinados. \n* E: Número de casos diagnosticados por la misma enfermedad en el mismo periodo y área. Es diferente de la tasa de mortalidad en que se da la proporción de muertes por una o varias enfermedades entre una población general en un periodo, que pueden estar afectados o no por la enfermedad, y por ello más fácil de establecer. Si es por una causa es la tasa de mortalidad específica y si es por todas las causas es tasa bruta de mortalidad.

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In epidemiology, a case fatality rate (CFR) — or case fatality risk, case fatality ratio or just fatality rate — is the proportion of deaths within a designated population of "cases" (people with a medical condition), over the course of the disease. A CFR is conventionally expressed as a percentage and represents a measure of risk. CFRs are most often used for diseases with discrete, limited time courses, such as outbreaks of acute infections. For example: Assume 9 deaths among 100 people in a community all diagnosed with the same disease. This means that among the 100 people formally diagnosed with the disease, 9 died and 91 recovered. The CFR, therefore, would be 9%. If some of the cases have not yet resolved (either died or recovered) at the time of analysis, this could lead to bias in estimating the CFR. A mortality rate — often confused with a CFR — is a measure of the number of deaths (in general, or due to a specific cause) in a population, scaled to the size of that population, per unit of time. (For example, a rate of 50 deaths per 10,000 population in a year resulting from diabetes. The mortality rate, therefore, would be 50:10,000 or 5:1,000.) Technically, CFRs are actually risks (or "incidence proportions") and take values between 0 and 1. They are not rates, incidence rates, or ratios (none of which are limited to the range 0-1). If one wants to be very precise, the term "case fatality rate" is incorrect, because the time from disease onset to death is not taken into account. Nevertheless, the term case fatality rate (and the abbreviation "CFR") is often used in the scientific literature.

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