Guidance

Emerging infections: how and why they arise

Updated 5 January 2023

How infections emerge

Between 60% and 80% of emerging infections are derived from animal sources.

Infectious disease emergence involves 2 main steps:

1. An agent is introduced into a new host population.
2. The agent is established and transmitted within the new host population.

Infections have been emerging for thousands of years as interactions between humans, animals and their environments have increased and changed.

Reasons for emergence

Disease emergence results from interactions between microbes and humans. These can be complex and multi-factorial. For example, population movement may result in the spread of a previously localised infection to a city where there is poor public health infrastructure, resulting in its establishment in the wider population. The city may then act as a source for further spread.

Factors involved in emergence are outlined below.

Microbial adaptation and change

Microbes continually adapt to the environment in which they live. The changes that result may enable them to evade the human immune system or provide them with a new ability to invade human or animal cells. They can also develop resistance to drug treatments, such as antibiotics, or undergo changes that result in vaccines being less effective.

Ecological changes

Ecological changes, including those due to agricultural or economic development, are among the most frequently identified factors in disease emergence.

Changes in the way land is used can result in humans coming into contact with animals and insect vectors, including:

• urbanisation
• agricultural practices
• deforestation
• reforestation
• dam building
• irrigation

These changes can increase the risk of human exposure to a potentially infectious disease.

Climate change can have a direct impact on how diseases are spread; either through effects on microbes and vectors (for example, a warmer climate may provide more favourable conditions in new geographical areas for insects, such as ticks and mosquitoes, which can carry and transmit disease) or because we may modify our behaviour in response to climate change.

Transmission of infectious diseases may also increase as a result of short-term weather conditions (for example, drought or flooding affecting cholera outbreaks) or appear at particular times of the year (for example, influenza in the winter).

Human demographics and behaviour

Activities such as sexual contact, drug use or outdoor recreational activities can result in increased exposure to an infectious microbe. Infections can then rapidly spread, especially in more heavily populated areas or where there are population movements or upheavals.

‘Bush meat’ harvested from previously remote forested areas remains a risk for animal-derived emerging infections. Other factors driving emergence include poverty, social inequality and lack of political will.

Technology and healthcare

Some diseases emerge as a direct consequence of technological change. New medical treatments and procedures like blood transfusion and organ transplantation, including xenotransplantation (the use of animal tissues or organs in humans), have created new ways for microbes to spread and infect people.

Travel, trade and industry

The speed and ease with which humans, animals and other goods can be transported around the world has made it much easier to spread both microbes and the vectors or vehicles that can transmit them.

Changes in food processing and packaging, and animal farming (including the widespread use of antibiotics in farm animals) also provide the potential for new diseases to emerge and spread.

Breakdown in public health measures

The spread of existing infections and the emergence of new ones in many areas of the world can be caused by poor sanitary conditions, hygiene and a lack of clean drinking water. Breakdown of public health measures, such as spraying mosquitoes (vector control) or vaccination programmes, can result in the re-emergence of previously controlled diseases.

Susceptibility to infection

The human body has many defences against infection. These defences can become weakened through disease, increasing age or other influences (for example malnutrition) making individuals more vulnerable to infection. Low vaccine uptake may result in populations becoming more susceptible to infection.

Emerging infections in humans between 2003 and 2022

The following timeline and map show newly identified emerging infections and notable outbreaks of diseases in humans in new areas between 2003 and 2022. The information is also presented in the table below.

Timeline of newly identified emerging infections and notable outbreaks of diseases in humans in new areas between 2003 and 2022

Global map of newly identified emerging infections and notable outbreaks of diseases in humans in new areas between 2003 and 2022

Table of newly identified emerging infections and notable outbreaks of diseases in humans in new areas between 2003 and 2022

Year	Disease	Location	Category
2022
2022	Langya henipavirus	China	Newly identified emerging infection
2022	Avian influenza A(H3N8)	China	Newly identified emerging infection
2022	Avian influenza A(H5N1)	Spain	Disease identified in new area
2022	Avian influenza A(H5N1)	UK	Disease identified in new area
2022	Avian influenza A(H5N1)	USA	Disease identified in new area
2022	Dengue	São Tomé and Príncipe	Disease identified in new area
2022	Marburg virus	Ghana	Disease identified in new area
2022	Mpox (monkeypox) virus	Global	Disease identified in new area
2021
2021	Alto Paraguay virus	Argentina	Newly identified emerging infection
2021	Avian influenza A(H5N8)	Russia	Newly identified emerging infection
2021	Avian influenza A(H10N3)	China	Newly identified emerging infection
2021	Yezo virus	Japan	Newly identified emerging infection
2021	Avian influenza A(H5N1)	India	Disease identified in new area
2021	Avian influenza A(H5N6)	Laos	Disease identified in new area
2021	Marburg virus	Guinea	Disease identified in new area
2020
2020	Guinea worm sp. nov.	Vietnam	Newly identified emerging infection
2020	Chikungunya	Chad	Disease identified in new area
2020	Oropouche virus	French Guiana	Disease identified in new area
2020	Severe fever with thrombocytopenia syndrome	Thailand	Disease identified in new area
2019
2019	SARS-CoV-2	Global	Newly identified emerging infection
2019	Avian influenza A(H5N1)	Nepal	Disease identified in new area
2019	Dengue	Afghanistan	Disease identified in new area
2019	Rift Valley fever	Central African Republic	Disease identified in new area
2019	Tick-borne encephalitis	UK	Disease identified in new area
2019	West Nile virus	Germany	Disease identified in new area
2018
2018	Avian influenza A(H7N4)	China	Newly identified emerging infection
2018	Guinea worm	Angola	Disease identified in new area
2017
2017	Rat hepatitis E	China	Newly identified emerging infection
2017	Lyme disease	Nepal	Disease identified in new area
2016
2016	Ntwetwe virus	Uganda	Newly identified emerging infection
2016	Crimean-Congo haemorrhagic fever	Spain	Disease identified in new area
2016	Lassa fever	Togo	Disease identified in new area
2015
2015	Zika virus	Brazil	Disease identified in new area
2014
2014	Chikungunya	Brazil	Disease identified in new area
2014	Bourbon virus	USA	Newly identified emerging infection
2014	Avian influenza A(H5N6)	China	Newly identified emerging infection
2014	Lassa fever	Benin	Disease identified in new area
2014	Ebola virus	Guinea, Liberia, Sierra Leone	Disease identified in new area
2013
2013	Avian influenza A(H7N9)	China	Newly identified emerging infection
2013	Avian influenza A(H10N8)	China	Newly identified emerging infection
2013	Colpodella spp. Heilongjiang (HLJ)	China	Newly identified emerging infection
2013	Sosuga virus	Uganda, South Sudan	Newly identified emerging infection
2013	Variegated Squirrel Bornavirus 1 (VSBV-1)	Germany	Newly identified emerging infection
2013	Chikungunya	Caribbean	Disease identified in new area
2012
2012	Middle East respiratory syndrome coronavirus (MERS-CoV)	Middle East	Newly identified emerging infection
2012	Mojiang paramyxovirus	China	Newly identified emerging infection
2012	Severe fever with thrombocytopenia syndrome	Japan, South Korea	Disease identified in new area
2011
2011	Influenza A(H3N2)v	USA	Newly identified emerging infection
2011	Plasmodium cynomolgi	Malaysia	Newly identified emerging infection
2011	Lassa fever	Ghana	Disease identified in new area
2010
2010	Cholera	Haiti	Disease identified in new area
2009
2009	Bas-Congo virus	DRC	Newly identified emerging infection
2009	Candida auris sp. nov.	Japan	Newly identified emerging infection
2009	Heartland virus	USA	Newly identified emerging infection
2009	Influenza A(H1N1)	Global	Newly identified emerging infection
2009	Severe fever with thrombocytopenia syndrome	China	Newly identified emerging infection
2009	Lassa fever	Mali	Disease identified in new area
2008
2008	Lujo virus	South Africa and Zambia	Newly identified emerging infection
2008	Crimean-Congo haemorrhagic fever	Greece	Disease identified in new area
2007
2007	Chikungunya	Italy	Disease identified in new area
2005
2005	Human T-lymphotropic virus (HTLV) 3	Cameroon	Newly identified emerging infection
2005	HTLV 4	Cameroon	Newly identified emerging infection
2003
2003	Chapare virus	Bolivia	Newly identified emerging infection