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# Case fatality rate

- Measure

Of the people who get a disease, how many die of it? It measures how severe the disease is, not how much it affects the whole population.

Updated Nov 5, 2026

## Key points

- Case fatality = deaths from the disease among the cases / number of cases × 100.
- It is a proportion (sometimes called the case fatality ratio), although by tradition it is called a rate.
- A very lethal but rare disease can cause fewer deaths in the population than a mild but common one.
- If more mild cases are found (more testing), the apparent case fatality falls even though the disease has not changed.
- In an outbreak it is calculated with the attack rate: one says how many fall ill, the other how many of them die.

On this page

- The basics Key points
- What it is
- Formula and reading
- Worked example
- Case fatality versus mortality rate
- Markers
- How to read it
- Common pitfalls
- Memory aid

- Study Test yourself

- Connections Connections

## What it is

The case fatality rate is the proportion of cases of a disease who die of it within a defined period. Its denominator is the cases, not the population, so it measures the severity of the disease. The mortality rate divides the same deaths by the whole population and measures the impact on the community.

## Formula and reading

- Case fatality (%) = (deaths among the cases / total cases) × 100.
- Disease mortality rate = (deaths from the disease / population) × 10ⁿ.
- A useful link: mortality ≈ incidence × case fatality, over the same period.
- The follow-up period must be long enough for the deaths of the included cases to occur.

## Worked example

- A made-up rural district of 50,000 people has two outbreaks in a year: disease A with 250 cases and 5 deaths; disease B with 20 cases and 4 deaths.
- Case fatality of A: 5 / 250 = 0.02, that is, 2%.
- Case fatality of B: 4 / 20 = 0.20, that is, 20%: B is ten times more lethal.
- Mortality from A: 5 / 50,000 × 100,000 = 10 per 100,000 population.
- Mortality from B: 4 / 50,000 × 100,000 = 8 per 100,000 population.
- Reading: B kills more of its patients, but A causes more deaths in the community because it makes many more people ill.

## Case fatality versus mortality rate

| Feature | Case fatality | Mortality rate |
| --- | --- | --- |
| Denominator | Cases of the disease | Total (mid-period) population |
| Question it answers | How severe is the disease? | How heavy is it for the community? |
| Unit | Percentage of cases | Per 1,000 or per 100,000 population |
| Changes when | Treatment improves or more mild cases are found | Frequency or severity changes |

## Markers

- CFR
- Case fatality
- % of cases

## How to read it

- Public health surveillance tracks the case fatality of severe events (for example, severe forms of vector-borne diseases) as an indicator of the quality of care.
- A rising case fatality in a hospital may point to delays in seeking or giving treatment, not only to a more aggressive strain.

## Common pitfalls

- Confusing case fatality with mortality: the denominator changes and so does the question.
- Calculating it early in an outbreak, when many cases have not yet finished their course.
- Comparing case fatality between places that look for cases with different intensity.

## Memory aid

Case fatality: out of the sick. Mortality: out of everyone who lives there.

## Test yourself

Think of the answer, then tap the question to see it.

## What is the denominator of the case fatality rate?

The number of cases of the disease in the period; the numerator is the deaths among those cases.

## An outbreak has 400 cases and 12 deaths. What is the case fatality?

12 / 400 = 0.03, that is, 3%.

## Why can apparent case fatality fall when diagnostic testing increases?

Because many mild cases that were missed before join the denominator, while deaths change little.

## Can a very lethal disease have a low mortality rate?

Yes, if it is rare: few people fall ill, so few die relative to the whole population.

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## Connections

## Related 5

- [Mortality rates: crude and specific](https://durgical.com/en/epidemiology/mortality-rates-crude-and-specific/)
- [Attack rate](https://durgical.com/en/epidemiology/attack-rate/)
- [Incidence](https://durgical.com/en/epidemiology/incidence/)
- [Outbreak investigation](https://durgical.com/en/epidemiology/outbreak-investigation/)
- [Public health surveillance](https://durgical.com/en/epidemiology/public-health-surveillance/)

[← All of epidemiology](https://durgical.com/en/epidemiology/)

Educational content on epidemiological and statistical methods; the examples use made-up numbers to teach the calculation. Not a clinical or legal guide: current Colombian rules are in the official sources.

Sources: [Principles of Epidemiology | Lesson 3 - Section 3](https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/lesson3/section3.html) (Centers for Disease Control and Prevention, 2012); [Principles of Epidemiology | Lesson 3 - Section 2](https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/lesson3/section2.html) (Centers for Disease Control and Prevention, 2012).

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Representative model, for educational purposes only.

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