Disease is an abnormal condition in any living organism that causes disorder in the structure and impairs the normal functioning of all or a part of the body with distinctive signs and symptoms.

**In epidemiology and public health, the adverse effects of such health outcomes are observed and quantified through various measures known as measures of disease frequency. **

It determines the frequency of occurrence of any disease or health events. Measuring the frequency of a disease and understanding how it varies over time and among different population subgroups is crucial for identifying the causes of the disease and developing effective preventive methods.

In epidemiological research, the three levels of response include describing the dissemination of the health outcome concerning the characteristics of the population and its frequency occurrence over time, identifying and establishing the determinants (factors) of the disease, and determining the impact of the proposed measures in relation to the disease for the treatment. Choosing the appropriate research measures at each response level is essential, depending on the research question and available data. Several measures of disease frequency are as follows:

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**Counts**

It is the fundamental measure of disease frequency and involves counting the number of people with the disease in a specific period of time. **For example**, the total count of cholera cases in 2021 in Biratnagar city is 160, and in 2022 is 170. Although the total count of the population with disease is essential in determining the trends, surveillance, and occurrence of health events, the information provided by this measure is rather limited.

**Ratio, Proportion and Rates**

The comparison between two independent quantities expressed as a fraction is known as a ratio. The numerator and denominator are mutually exclusive, indicating one quantity is greater than the other. It is used both as a descriptive measure and an analytical tool. **For example**, the ratio of male and female population affected by diabetes is 2/1 and expressed as 2:1.

A type of ratio in which the numerator is the part of the denominator is known as proportion. It is a descriptive measure in epidemiology that compares a part of the quantity to the whole. It can be expressed as a decimal, fraction, or percentage. **For example**, the number of diabetic men is 150 and the total number of men is 560. Then, the proportion of diabetic men is (150/560)*100 = 26.78%.

Rate is defined as the frequency at which an event occurs in a specified period in a population at risk. It is the measure of risk and is useful for comparing disease frequency across various locations, time periods, or population groups with varied characteristics and potentially different-sized populations.

**Incidence and Prevalence**

Any deviation, either subjective or objective, from the state of physical and psychological well-being is known as morbidity. It includes conditions such as disease, injury, and disability. In this context, the term refers to the number of people affected, the periods of illness experienced by the population, or the duration of these illnesses. Incidence and prevalence are the most common measures of morbidity frequency. These parameters are often confused as similar, therefore their key differences must be understood.

**Incidence**

The incidence of a disease indicates the rate of occurrence of new cases of disease within a specific population over a period of time. It is determined by dividing the number of new cases of disease in a specific time period (often a year) by the total number of initially disease-free populations. **For example**, the annual incidence of meningococcal disease in Auckland, New Zealand is 16.9/100,000 people.

Incidence Proportion, Cumulative Incidence, or Risk is the proportion of the population who were initially disease-free with the probability of developing disease and injuries over a specified period of time. It is an estimation of the risk of acquiring a disease in an individual within the population. Similarly, incidence is also expressed in terms of Incidence Rate, also known as Person-time rate or Incidence Density. It is the rate of the number of new cases of disease to the sum of the total time at risk of disease for the population.

**Prevalence**

The prevalence of a disease indicates the proportion of existing cases of disease within a specific population in a given period of time. It is determined by dividing the current number of cases in a specific population at a given time by the total number of populations examined. **For example**, Out of 3000 people examined, 735 were currently affected by HIV AIDS at the given time of the study, then the prevalence of HIV AIDS is calculated as 735/3000 = 0.245. When expressed as a percentage, the ratio is multiplied by 100, thus, the prevalence of AIDS was found to be 24.5% at that time.

Furthermore, point prevalence refers to the proportion of the population affected by a particular disease at a specific point in time, while period prevalence refers to the proportion of the population affected by a particular disease over a period of time.

**Difference between Incidence and Prevalence**

Incidence | Prevalence |

The rate or proportion of new cases within a population at a specific time period. | The proportion of existing cases within a population at the given time. |

It is a dynamic measure of disease frequency. | It is a static measure of disease frequency. |

It includes new cases only. | It includes new and pre-existing cases. |

It is used as a measure of risk factors and investigation of causes and determinants of disease occurrence. | It is used as a measure to describe disease burden for chronic diseases with long duration and dates of onset. |

Cumulative Incidence ranges from 0-1 and Incidence Rate ranges from 0 to infinity. | It ranges from 0-1. |

Cumulative Incidence is interpreted as probability and Incidence Rate is interpreted as the reciprocal of time at risk. | Prevalence is interpreted as proportion. |

**Mortality Rates**

Mortality simply refers to the frequency of deaths from a disease. The average number of years a newborn is expected to live is referred to as life expectancy at birth, considering the age-specific mortality rates are unchanged. Life expectancy is the key indicator of general mortality in a population taking into account public health facilities and health care systems. The total number of deaths from a particular disease in a defined population in a specific time interval is defined as the mortality rate. **For example**, in a population of 500 people, 200 deaths occurred due to typhoid, then the mortality rate is given by 200/500 = 0.4 or 40%.

The various measures of mortality based on the characteristics and demography of the population are as follows:

- Crude Mortality Rate/Crude Death Rate
- Case-Specific Death Rate
- Neonatal Mortality Rate
- Post-neonatal Mortality Rate
- Infant Mortality Rate
- Maternal Mortality Rate
- Proportionate Mortality
- Death-to-case ratio

**Case Fatality Rates**

In epidemiology, the ratio of deaths due to a specific disease to the total number of diagnosed cases of that disease within a given period of time is known as the case fatality rate. **For example**, in a population of 1,000 people, 500 have been diagnosed with a specific disease and 150 have died from the same disease. In this case, the case fatality rate is 150/500 = 0.3 or 30%.

It is the estimation of the risk of dying from a disease and is used to assess the efficiency of new treatments. With improved treatments, the case fatality rates decrease. Various parameters including the causative factor of disease, host, environmental conditions, and available treatments vary the case fatality rates among populations and time periods.

**Attack Rates**

In epidemiological studies, attack rates or attack risk is a term used in outbreak settings with relatively shorter periods of observation. It is the proportion of people suffering from a disease based on case definition (clinical symptoms or serology) within a population at risk at the beginning of the defined time interval. It is used in the investigation of outbreaks, generally acute to help identify the exposures contributing to the disease such as consumption of specific food. Various types of attack rates are as follows:

- Overall Attack rate
- Food-specific attack rate
- Secondary attack rate

**References**

- https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/lesson3/section1.html
- https://www.blackwellpublishing.com/specialarticles/jcn_9_188.pdf
- https://s4be.cochrane.org/blog/2020/11/06/prevalence-vs-incidence-what-is-the-difference/
- https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/lesson3/section2.html
- Chapter 2. Quantifying disease in populations | The BMJ. (n.d.). Retrieved from https://www.bmj.com/about-bmj/resources-readers/publications/epidemiology-uninitiated/2-quantifying-disease-populations
- https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/lesson3/section2.html
- https://www.britannica.com/science/mortality-demography
- https://www.britannica.com/science/attack-rate
- https://archive.cdc.gov/www_cdc_gov/csels/dsepd/ss1978/lesson3/section3.html
- Noordzij, M., Dekker, F. W., Zoccali, C., & Jager, K. J. (2010). Measures of Disease Frequency: Prevalence and Incidence. Nephron Clinical Practice, 115(1), c17–c20. doi:10.1159/000286345