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

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

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