Public Health Definitions
(with acknowledgement of the US Centers for Disease Control and Prevention, Public Health 101)

​Public Health
Public health is “the science and art of preventing disease, prolonging life, and promoting health through the organized efforts and informed choices of society, organizations, public and private communities, and individuals,” according to Charles-Edward Winslow, a public health expert in the early 20th century.  Learn more here. ​

Epidemiology
Epidemiology is the study of the distribution and determinants of health-related states or events in specified populations, and the application of this study to the control of health problems.  It is the method used to find the causes of health outcomes and diseases in populations.  Learn more here. 

Levels of Disease
There are several different descriptions of disease distribution.  Learn more here. 

• ​Endemic Disease  - ​The persistent presence of a disease or infectious agent in a population within a specific geographic area (e.g. Lyme disease in New England)
• Epidemic Disease - ​An increase in the number of cases of a disease above what is normally expected in that population in that area (e.g. Ebola virus disease in sub-Saharan Africa) 
• Pandemic Disease​ - An epidemic that has spread over several countries or continents, usually affecting a large number of people (e.g. COVID-19, 1918 influenza).  Pandemics usually arise from diseases that spread quickly and effectively through populations. 
• Sporadic Disease​ - A disease that occurs infrequently and irregularly 
• Cluster - ​An aggregation of cases grouped in place and time that are, or suspected to be, greater than the number expected 

Important Concepts

• Symptom - what a person with a disease feels or experiences (e.g. headache, feverish, sore throat)
• Sign of disease - what is objectively apparent in association with disease (e.g. measured fever, low blood pressure, red throat)
• Infection - the invasion of the body or a tissue by an infectious agent, with or without symptoms or signs of the infection
• Disease - in the case of infection, the signs and symptoms that may result from the infection
• Incubation period - the time from acquiring infection to the onset of symptoms (varies by disease)
• Infectious period - the period of time in the course of infection during which the person with infection can transmit the infection (varies by infection)
• Inoculum size - the amount or dose of the infectious agent (bacterium, virus, fungus, parasite) that is likely to cause infection (varies by infectious agent)​

Outbreak Patterns
There are several types of outbreaks. Two types are common-source outbreaks, where a group of people are exposed from the same source, and propagated outbreaks, where the disease is spread most commonly person-to-person.  Learn more here.

• Common-Source Outbreak​ - A group of persons are all exposed to an infectious agent or toxin from the same source 
• Point-Source Outbreak​ - the group is exposed to the same source over a relatively brief period; everyone who becomes ill does so within one incubation period 
• Continuous Common-Source Outbreak​ - the group is repeatedly exposed to the same source over one or several incubation periods
• Intermittent Common-Source Outbreak​ -  the group is exposed to the same source at irregular intervals 
• Propagated Outbreak​ - a disease is transmitted from one person to another, usually by direct person-to-person contact (e.g. syphilis). Transmission may also be vehicle-borne (e.g. transmission of HIV by sharing needles) or vector-borne (e.g. transmission of yellow fever by mosquitoes). Cases usually occur over more than one incubation period 
• Mixed Outbreak​ -  Mixed epidemics feature both common- source and propagated epidemics. Typically, you’ll see the pattern of a common-source outbreak followed by a secondary person-to-person spread 

Types of Infections
Many infectious diseases — such as pneumonia, meningitis and diarrhea — are caused by either bacterial infections or viruses.  Learn more here. 

• Bacteria - single-celled microorganisms that thrive in many different types of environments.  Some varieties live in extremes of cold or heat. Others make their home in people’s intestines, where they help digest food. Most bacteria cause no harm to people, they are part of the natural healthy "microbiome", but there are exceptions. Some infections caused by bacteria include: strep throat, tuberculosis and urinary tract infections. Use of antibiotics, in particular use that may be inappropriate, has helped create bacteria that are resistant to treatment with different types of antibiotic medications. 
• Viruses - even smaller than bacteria and require living hosts cells to multiply. Otherwise, they can’t survive. When a virus enters your body, it invades some of your cells and takes over the cell machinery, redirecting it to produce the virus. Diseases caused by viruses include: chickenpox, acquired immunodeficiency syndrome  (AIDS) and common colds, among other things. There are some drugs that can fight viral infections (antivirals), but viruses are not affected by antibiotics that inhibit or kill bacteria.  

Public Health Organizations
There are public health organizations at multiple levels of society. 

• Local health departments - public health was first organized at the local level (for example the Boston Board of Health was founded in 1799).  Most public health around the world is done at the local level - towns, cities, counties and districts.  Learn about local health departments here and here.
• State and provincial health departments - the next higher level of public health is at the level of state or province.  In the United States, public health authority is vested with the state, with the federal government in a supporting role.  Every state and territory has a health department.  The Massachusetts Board of Health was created in 1869 and ultimately became the Massachusetts Department of Public Health (MDPH).  Learn more about one state health department here.
• The United States Public Health Service (USPHS) - under the Assistant Secretary of Health of the US Department of health and Human Services, the USPHS consists of multiple agencies.  The USPHS origins were in marine hospitals established to care for mariners in 1798 and it became the PHS in 1912.  Both Centers for Disease Control and Prevention​ and National Institutes of Health​ are part of the USPHS, along with the Food and Drug Administration, Substance Abuse and Mental Health Services Administration, Indian Health Service, Agency for Toxic Substances and Disease Registry, Health Resources and Services Administration and the Agency for Healthcare Research and Quality. The Surgeon General of the United States leads the uniformed, commissioned corps of the PHS.  
  • Centers for Disease Control and Prevention​ (CDC) - the CDC was founded on July 1st, 1946, to prevent malaria from spreading across the nation. Today, the CDC is “one of the major operating components of the Department of Health and Human Services and is recognized as the nation’s premiere health promotion, prevention, and preparedness agency,” and has multiple centers/institutes/offices.  Learn more here.
  • National Institutes of Health​ (NIH)- NIH was founded in 1887 as a one-room Hygienic Laboratory within the Marine Hospital Service. Today, it serves as the nation’s medical research agency. The NIH is responsible for leading edge medical research and for funding medical research across the country.  Learn more here. 
• WHO: World Health Organization​ - The WHO was founded on April 7th, 1948, as the lead international health agency in the United Nations system. The WHO operates on the principle that “all people should enjoy the highest standard of health, regardless of race, religion, political belief, economic or social condition. It is governed by the World Health Assembly, which is made up of delegates from 194 member states. Today, April 7th is known as World Health Day – a day to celebrate the work of nurses and midwives and remind world leaders of the critical role they play in keeping the world healthy. Learn more here.