Emerging Pathogens

An "emerging pathogen" can be defined as the causative agent of an infectious disease whose incidence increases after its emergence in a new host population or whose incidence increases in an existing host population as a result of long-term changes in the underlying epidemiology.

One possible source of emerging pathogens is other host species (a "reservoir") in which the pathogen is already established. Switching from one host species to another (species switching) has led to some of the most devastating known disease epidemics, including the ongoing HIV/AIDS pandemic in human communities worldwide, the decimation of the European rabbit population by myxomatosis in the mid-20th century, rinderpest with spread to African ruminants in the late 19th century, and more recently widespread mortality of North Sea seals due to canine distemper.

Species changes have also caused widespread epidemics of plant pathogens in cultivated plants (e.g. late blight in cultivated potatoes) and wild plants (e.g. the almost complete extinction of American chestnut trees due to chestnut blight).

Emerging pathogens affecting humans include:

• Outbreaks of new diseases that were previously unknown
• Known diseases that are now spreading rapidly in the number of cases or in the number of areas where people are sick
• Known infectious diseases that are persistent and cannot be controlled

In detail. Emerging diseases include:

• Coronavirus infections such as COVID-19, SARS and MERS
• HIV infections
• Lyme disease
• Escherichia coli (E. coli) O157:H7
• Hantavirus
• Dengue fever
• West Nile virus
• Zika virus

Re-emerging Emerging Pathogens are those that re-emerge after a sharp decline. This may be due to problems with public health measures for diseases that were previously under control. They can also occur when new strains of known diseases emerge. People's behavior can cause diseases to re-emerge. For example, declining vaccination rates and failure to maintain proper vaccine storage and handling temperatures to maintain efficacy have led to the re-emergence of measles in some areas. The overuse of antibiotics has also meant that some bacteria and viruses have adapted to drugs. This means that they cannot be killed as easily. This has meant that diseases that were once under control and treatable are now infecting more people. Re-emerging diseases include:

• Malaria
• tuberculosis
• cholera
• pertussis
• influenza
• Pneumococcal disease
• gonorrhea
• Fungal diseases (e.g. various Trichophyton and yeast species)

Travelers should be aware that some diseases that are under control in European countries may be active in other countries. The risk is even greater for travelers who fall ill in a country where treatment options for these diseases are limited. All people planning a trip should obtain detailed information about the health risks in the countries they are traveling to.

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