An infectious disease (also transmissible or communicable disease) is an illness that is caused by the invasion of a host by agents (mostly bacterial and viral pathogens) whose activities harm the host’s tissues and can be transmitted to other individuals. Although infectious diseases are no longer the leading cause of death globally, some infectious illnesses (especially lower respiratory infections) still rank highly with regard to mortality. This burden is also contributed by emerging and reemerging infectious diseases.

Emerging infectious diseases can be defined in three ways: (1) new diseases that are occurring in humans for the first time (these are difficult to establish and are mostly rare, e.g. COVID-19); (2) diseases that have occurred previously, affecting only small numbers of people in some places (e.g. AIDS and Ebola hemorrhagic fever); and (3) diseases that have existed throughout human history but were just recently recognized as unique diseases as a result of an infectious agent (e.g. Lyme disease and gastric ulcer). Several studies have shown that modernization and changes within the environment are related to the emergence of many infectious diseases. For example, Lyme disease, hantavirus pulmonary syndrome (HPS), and Lassa fever have all emerged as a result of humans coming into contact with the causative agents of these diseases in recent times. Factors related to the emergence of infectious diseases such as Legionnaire disease and hemolytic uremic syndrome include changing technologies such as air conditioning systems for the former disease and mass food production for the latter. In the past three to four decades, the following emerging infectious diseases have been some of the most active, causing high morbidity and mortality rates: Ebola hemorrhagic fever, AIDS, gastric ulcer, Lassa fever, and COVID-19. The latest emerging infectious disease is the Coronavirus disease (COVID-19).

Re-emerging infectious diseases are diseases that were formerly major public health concerns around the world or in specific countries or regions that declined considerably, but are again becoming a public health concern for a substantial part of the population (e.g. malaria, diphtheria, rabies, pertussis, measles, and tuberculosis). Some infectious disease specialists consider reemerging infectious diseases as a subcategory of emerging diseases. The reemergence of infectious diseases occurs for several reasons. Firstly, there may be a slack/breakdown in public health measures that were once used to control the disease. There may be an evolution of new strains of the causative agents; causing the pathogens to acquire drug (antibiotic) resistance. Additionally, the reemergence of diseases could be a result of inadequate vaccination of the population. When the proportion of immune individuals in a population drops below a particular threshold, the introduction of the pathogen into the population leads to an outbreak of the disease.

References

1. National Institutes of Health (US); Biological Sciences Curriculum Study. NIH Curriculum Supplement Series [Internet]. Bethesda (MD): National Institutes of Health (US); 2007. Understanding Emerging and Re-emerging Infectious Diseases. Available from: https://www.ncbi.nlm.nih.gov/books/NBK20370/
   

2. John Hopkins medicine. Emerging Infectious disease. https://www.hopkinsmedicine.org/health/conditions-and-diseases/emerging-infectious diseases#:~:text=Emerging%20diseases%20include%20HIV%20infections,been%20on%20a%20significant%20decline.
   

3. Fauci, Anthony S. MD Emerging and Reemerging Infectious Diseases: The Perpetual Challenge, Academic Medicine: December 2005 – Volume 80 – Issue 12 – p 1079-1085.