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Public Health

Vetted resources and information on current public health events.

Disease Occurrence Defintions

Sporadic

Diseases that occur infrequently or irregularly

  • These are usually rare or eradicated diseases so that a single case warrants an investigation (polio, measles, etc.) 
Endemic

Baseline level of a particular disease

  • This is not necessarily a desired level (a disease can be considered endemic at very high levels of occurrence)
  • Typically has a constant presence in a population, or predicable cycles (eg. seasonal), in a geographic area
  • Expected level of disease without mitigations
Outbreak

Disease in the community rises above the expected (baseline) level

  • This is within a limited or specific geographic area, such as neighborhood, city, county, state
  • Can also be caused by a specific event or gathering in which cases all stem from
Epidemic

Disease in a widespread geographic area rises above the expected (baseline) level

  • Usually involves entire regions or countries
Pandemic

Disease epidemic spread across several countries or continents

  • Usually involves massive numbers of people and infections

U.S. Disease Surveillance Resources

National Notifiable Diseases Surveillance System (NNDSS)

The NNDSS is a nationwide collaboration that enables all levels of public health (local, state, territorial, federal, and international) to share health information to monitor, control, and prevent the occurrence and spread of state-reportable and nationally-notifiable infectious and some non-infectious diseases and conditions. The CDC administers NNDSS in collaboration with CSTE. 

 
What is Case Surveillance?

Case surveillance is foundational to public health practice. It helps us to understand diseases and their spread and determine appropriate actions to control outbreaks. Case surveillance occurs each time public health agencies at the local, state, or national levels collect information about a case or person diagnosed with a disease or condition that poses a serious health threat to Americans. These diseases and conditions include:

  • Infectious diseases, such as coronavirus disease 2019 (COVID-19)
  • Foodborne outbreaks, such as E.coli
  • Noninfectious conditions, such as lead poisoning

Case surveillance starts at local, state, and territorial public health departments. Local laws and regulations specify which diseases and conditions must be reported. The health departments work with healthcare providers, laboratories, hospitals, and other partners to get the information needed to monitor, control, and prevent these reportable diseases and conditions in their communities.

 
Nationally-Notifiable Conditions

Health departments also notify CDC about certain conditions so we can track them for the whole country. CDC monitors about 120 of these notifiable diseases and conditions at the national level. This important step helps protect the health of individual communities and the nation. Following standard case definitions, case surveillance captures information that public health officials can use to understand where diseases are occurring, how they can be prevented, and which groups are most heavily impacted. 

The CDC conducts case surveillance through the National Notifiable Diseases Surveillance System (NNDSS). The 120 diseases and conditions under surveillance through NNDSS including infectious diseases, bioterrorism agents, sexually transmitted diseases, and some non-infectious conditions. About 3,000 local public health departments send disease data to 60 regional public health departments (50 U.S. states, NYC, Washington DC, and 5 U.S. territories: American Samoa, Northern Marinara Islands, Guam, Puerto Rico, and the U.S. Virgin Islands), who then send the data to CDC. Through NNDSS, CDC receives and uses these data to keep people healthy and defend America from health threats.

2025 National Notifiable Conditions

 

Reportable vs Notifiable Diseases

While the terms reportable and notifiable are often used interchangeably, when it comes to public health and disease surveillance, there are clear differences between the two.

Reportable Diseases and Conditions Notifiable Diseases and Conditions
Each state or territory sets local laws and rules for which diseases and conditions must be reported to local and/or state health departments. The Council of State and Territorial Epidemiologists and CDC identify the list of notifiable diseases and conditions.
Healthcare professionals, laboratories, hospitals, and other providers must tell public health departments when a person is diagnosed with a reportable disease or condition. States voluntarily inform CDC when a person meets certain criteria to become a case.
Public health departments collect information about the person and how they became ill. Case records do not contain personally identifiable information.
This information is used to locate the source of an infection or outbreak and prevent spread. The CDC uses data to monitor, measure, and alert individual communities or the nation to outbreaks and other public health threats.
The list of reportable diseases and conditions can change every year. The list contains about 120 diseases and conditions and is updated every year.
NNDSS Data 

In January 2025, the CDC's Wide-ranging ONline Data for Epidemiologic Research (WONDER) platform completed its planned migration to a new Azure cloud environment. As a result of the WONDER migration, CDC has moved PDF and text versions of the weekly and annual NNDSS data tables to new locations on our website:

  • Annual data tables can be found on CDC Stacks, an online repository available to public health professionals, researchers, and the public.
  • Weekly tables can be found on CDC Stacks or data.cdc.gov, a publicly available data catalog of data resources.

However, CDC WONDER will continue to host the interactive NNDSS annual summary data query.

 

Global Disease Surveillance Resources