Disease outbreaks
A disease outbreak is the occurrence of disease cases in excess of normal expectancy. The number of cases varies according to the disease-causing agent, and the size and type of previous and existing exposure to the agent.
Disease outbreaks are usually caused by an infection, transmitted through person-to-person contact, animal-to-person contact, or from the environment or other media.
Outbreaks may also occur following exposure to chemicals or to radioactive materials. For example, Minamata disease is caused by exposure to mercury.
Occasionally the cause of an outbreak is unknown, even after thorough investigation.
Communicable disease outbreaks
Environmental factors influencing the spread of communicable diseases
Water, sanitation, food and air quality are vital elements in the transmission of communicable diseases and in the spread of diseases prone to cause epidemics.
A number of environmental factors influence the spread of communicable diseases that are prone to cause epidemics. The most important of these are:
- water supply
- sanitation facilities
- food
- climate.
A lack of safe water, inadequate excreta disposal facilities, poor hygiene, poor living conditions and unsafe food can all cause diarrhoeal diseases. These diseases are a major cause of suffering and death in an emergency situation.
Climate can affect disease transmission in a variety of ways. The distribution and population size of disease vectors can be heavily affected by local climate. Flooding after heavy rains can result in sewage overflow and widespread water contamination. In addition, there is some evidence to suggest that pathogens can be spread from one region to another along air streams or by wind.
WHO’s response to communicable disease outbreaks is led by the Epidemic and Pandemic Alert and Response Department.
WHO Key resources
- Communicable disease control in emergencies. A field manual (2005)
- Environmental health in emergencies and disasters: a practical guide (2002)
Chapters 10 and 11 - Practical guidelines for infection control in health care facilities. Chapter 4. Environmental management practices (2003)
- Vector-borne disease fact sheets
- Sanitation fact sheet
- Drinking-water fact sheet
Related activity
Disease outbreaks caused by chemicals
There are many examples of disease outbreaks that are caused by exposure to chemicals or toxins.
Tri-ortho-cresyl phosphate is a contaminant of medicines, food and drink. It is a neurotoxin that has caused a number of outbreaks of neurological illness including the so-called ‘Jamaican ginger paralysis’.
In 2001, a series of unexpected deaths in dialysis patients in six countries was eventually shown to be caused by the contamination of the dialysis machine filters with perfluoroisobutylene.
Investigating the cause of a disease outbreak requires careful epidemiological and toxicological investigation. In some cases it may take many years of research before the etiological agent and its mechanism of toxicity are identified.
Read more on Chemical Safety
Disease outbreaks of unknown etiology
Occasionally, an outbreak or cluster of disease is seen in a population for which the cause is unclear. These outbreaks may be due to a new or modified pathogen, a natural toxin, an initially undetected release of a chemical, or over-exposure to ionizing radiation from an unknown source. Careful history taking and epidemiological study may point to one or more possible causes that can then provide a focus for further, more specialized investigation.
Nodding disease, observed in Southern Sudan, is an example of a disease for which the etiological agent remains a mystery.
WHO key resources
Publications
The purpose of the WHO Manual for the Public Health Management of Chemical Incidents is to provide a comprehensive overview of the principles and roles...
The emergence of life-threatening infections such as severe acute respiratory syndrome (SARS) and re-emerging infectious diseases like plague and tuberculosis...
Communicable disease control in emergencies: a field manual
This manual is intended to help health professionals and public health coordinators working in emergency situations prevent, detect and control the major...
Environmental health in emergencies and disasters: A practical guide
The WHO Guide to sanitation in natural disasters (Assar, 1971) summarized the essential aspects of environmental health management in disasters. These...