incidence and incidents - the difference
 
Incidence is a proportion (usually of society in relation to a disease).
An incident is an event.
 

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Incidence

The noun incidence is very technical and quite rare (unless you study the spread of disease).  It relates to the chance of developing some new condition (usually a disease) within a specified time period.

Examples:

The chart 'Incidence of Flu by Area' shows the number of people with flu and flu-like illnesses who visited their doctor last week.

They suffer a higher incidence of measles as they did not invest in health-care systems to deliver vaccinations effectively.

Incidents

The word incidents is the plural of incident, which means an occurrence or an event.

Examples:

There is no evidence to link the two incidents at this time.

The police are investigating two incidents of someone shooting into occupied homes.

See also:

What are nouns?
What are verbs?
List of easily confused words