Incident = event, happening which attract general attention.

• The government regretted the incident.
• Several people were injured in the incident.
• Many such incidents go unreported and the guilty escape punishment.
• Two people were shot yesterday in two separate incidents.

Incidence = The rate, or frequency of a disease, crime or other undesirable things.

• Now there is an increased incidence of cancer among city dwellers.
• Records prove that there is a high incidence of heart disease in men over 50.
• High incidence of malaria has been reported in the area.
• This area has a high incidence of crime, disease and unemployment.