Incidence is defined as:

Incidence is defined as:

• A. The occurrence of new disease or death within a defined period, among those at risk.
• B. The total existing cases at a point in time.
• C. The ratio of deaths to cases.
• D. The overall prevalence.

Answer: (A)

Incidence is the rate at which new cases of a disease (or death from it) occur in a population during a defined period, among people who are at risk. It specifically tracks new events and requires a time component, so you can compare how fast the disease is appearing in different groups or over different time frames. This is different from prevalence, which counts all existing cases at a point in time (or over a period) regardless of when they started. It’s also not a measure of severity or lethality like a deaths-to-cases ratio; that describes how deadly the disease is among those who have it, not how often new cases appear. For example, if 50 people develop a disease in a population of 10,000 over one year, the incidence for that year is 50 new cases divided by 10,000 people, or 0.5% (often expressed as 50 per 10,000 person-years in a rate form). The idea of “at risk” ensures those who already have the disease (or are immune) aren’t counted as new cases.

Incidence is the rate at which new cases of a disease (or death from it) occur in a population during a defined period, among people who are at risk. It specifically tracks new events and requires a time component, so you can compare how fast the disease is appearing in different groups or over different time frames. This is different from prevalence, which counts all existing cases at a point in time (or over a period) regardless of when they started. It’s also not a measure of severity or lethality like a deaths-to-cases ratio; that describes how deadly the disease is among those who have it, not how often new cases appear. For example, if 50 people develop a disease in a population of 10,000 over one year, the incidence for that year is 50 new cases divided by 10,000 people, or 0.5% (often expressed as 50 per 10,000 person-years in a rate form). The idea of “at risk” ensures those who already have the disease (or are immune) aren’t counted as new cases.