Incidence is how often something happens in a group, over a period of time — especially illness or crime. Doctors talk about the incidence of flu; newspapers talk about the incidence of theft. It is a counting word for reports and textbooks, not for chat.

Incidence isIncidence（發生率）指的是 how often something happens in a group某件事在一個群體裡發生的頻率， over a period of time在一段時間內—— especially illness or crime尤其是疾病或犯罪。Doctors talk about醫生會討論 the incidence of fluflu（流感）的incidence； newspapers talk about報紙會討論 the incidence of thefttheft（竊盜）的incidence。It它 is a counting word for reports and textbooks是報告和教科書裡用的計數用字， not for chat不是聊天用的。

Though actually, students mix this up with incident almost every time, and that is nothing to be ashamed of: the two sit side by side on the page. An incident is one event: there was an incident on the train. Incidence is the rate: the incidence of accidents on that line has fallen. One is a thing that happened; the other is a number.

Though actually不過話說回來， students mix this up with incident almost every time學生幾乎每次都會把它跟incident搞混， and that is nothing to be ashamed of這沒什麼好丟臉的： the two sit side by side on the page這兩個字在字典裡就是緊挨在一起。An incident isAn incident是 one event一個單一事件： there was an incident on the trainthere was an incident on the train（火車上發生了一件事）。Incidence isIncidence指的是 the rate比率： the incidence of accidents on that line has fallenthe incidence of accidents on that line has fallen（那條線上意外的incidence下降了）。One is a thing that happened一個是發生過的事情； the other is a number另一個是一個數字。

Doctors also keep incidence apart from prevalence. Incidence is new cases. Prevalence is how many people have the illness right now, including old cases. Ten new colds this week is incidence; fifty people in town who currently have a cold is prevalence.

Doctors醫生 also keep incidence apart from prevalence也會把incidence跟prevalence（盛行率）分開來看。Incidence isIncidence指的是 new cases新的病例。Prevalence isPrevalence指的是 how many people have the illness right now現在有多少人得這個病， including old cases包括舊病例。Ten new colds this week這星期十個新的感冒病例， is incidence是incidence； fifty people in town who currently have a cold鎮上目前有五十個人正感冒著， is prevalence是prevalence。

There is another sense in science: the angle of incidence is the angle at which light hits a surface. That still comes from the old picture of something falling upon a thing.

There is another sense in science在科學裡還有另一個意思： the angle of incidencethe angle of incidence（入射角） is the angle at which light hits a surface指的是光線打到一個表面時的角度。That這 still comes from the old picture其實還是來自古老的畫面， of something falling upon a thing某個東西落在另一個東西上。

The illness-and-crime meaning is the one you need here.

The illness-and-crime meaning疾病和犯罪這個意思， is the one you need here才是你在這裡需要的。