Incidence is a proportion (usually of society in relation to a disease).
An incident is an event.
The nounincidence 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.
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.
The word incidents is the plural of incident, which means an occurrence or an event.
There is no evidence to link the two incidents at this time.
The police are investigating two incidents of someone shooting into occupied homes.