Understanding Emerging Pathogens: Definition and Classification

In 2009, a previously unknown fungal species was discovered in Japan. Within just over a decade, Candida auris spread to every continent except Antarctica, sparking alarm in healthcare facilities worldwide. This rapid global spread exemplifies the growing challenge of emerging pathogens-infectious agents that have recently appeared or are rapidly increasing in prevalence, geographic range, or both. Understanding these microbial threats is essential for protecting public health and preventing the next pandemic.

What defines an emerging pathogen
Viral emerging pathogens
Arboviruses as emerging threats
Bacterial emerging pathogens
Fungal emerging pathogens
Parasitic emerging pathogens
Prions as unique emerging threats
Why pathogens emerge
The public health challenge

What defines an emerging pathogen

Emerging pathogens are infectious diseases that have either newly appeared in a population or have existed but are rapidly increasing in incidence, geographic range, or severity. These threats encompass several distinct scenarios. Some pathogens are completely new discoveries, identified through advanced diagnostic tools. Others have existed in isolated regions but are now spreading to new geographic areas. Still others represent known diseases that have acquired new characteristics, such as drug resistance or increased virulence.

Approximately 13% of all recognized human pathogens are classified as emerging or reemerging. What makes these pathogens particularly concerning is their potential to cause severe illness, their ability to spread rapidly between people or from animals to humans, and their resistance to existing treatments. The COVID-19 pandemic demonstrated how a single emerging pathogen can disrupt societies, economies, and healthcare systems globally.

Viral emerging pathogens

Viruses represent the largest category of emerging pathogens. Of 208 recognized viral pathogens affecting humans, 77 are considered emerging or reemerging, making them significantly overrepresented compared to other pathogen types. RNA viruses are particularly prominent among emerging threats because their high mutation rates allow rapid adaptation to new hosts.

SARS-CoV-2, the virus responsible for COVID-19, emerged in late 2019 and quickly became a global pandemic. Similarly, Ebola virus has caused multiple outbreaks in Africa with high mortality rates. Zika virus, once considered a mild tropical disease, gained international attention when researchers linked it to severe birth defects. Influenza viruses continuously evolve, with strains like H5N1 and H7N9 demonstrating potential for pandemic spread.

Most emerging viral pathogens originate from animal reservoirs through a process called zoonotic transmission. An estimated 75% of all known emerging infectious diseases originate from some type of animal reservoir. When viruses jump from wildlife to humans, they can sometimes acquire the ability to spread from person to person, creating pandemic potential.

Arboviruses as emerging threats

Arboviruses-viruses transmitted by arthropod vectors like mosquitoes and ticks-represent a particularly important subset of emerging viral pathogens. Dengue virus, West Nile virus, and chikungunya have all expanded their geographic ranges in recent decades. Climate changes and international travel have created favorable conditions for these vector-borne diseases to establish themselves in previously unaffected regions.

Bacterial emerging pathogens

While bacteria comprise a smaller proportion of emerging pathogens compared to viruses, they pose significant challenges, particularly through antimicrobial resistance. Drug-resistant bacteria evolve when antibiotics are overused or misused, selecting for strains that can survive treatment.

Multidrug-resistant tuberculosis (MDR-TB) has emerged as a major global health concern. The bacterium Mycobacterium tuberculosis has developed resistance to multiple antibiotics, complicating treatment and requiring extended therapy with less effective drugs. Carbapenem-resistant Enterobacteriaceae (CRE) have developed resistance to carbapenem antibiotics, often considered the last line of defense against serious bacterial infections.

Healthcare-associated methicillin-resistant Staphylococcus aureus (MRSA) was responsible for approximately 94,360 serious infections in the United States in 2005, demonstrating how emerging resistant bacteria can become established threats within medical facilities. Food-borne bacterial pathogens like E. coli O157 and Salmonella strains have also evolved new characteristics that enhance their virulence or transmission.

Fungal emerging pathogens

Fungi have received less attention historically but represent growing threats, especially for immunocompromised individuals. Candida auris was first isolated in 2009 and has since emerged globally as a multidrug-resistant healthcare-associated pathogen. This yeast can cause severe infections with mortality rates ranging from 30% to 60%.

What makes Candida auris particularly alarming is its unusual characteristics among fungi. The pathogen can colonize human skin and persist on hospital surfaces for extended periods, facilitating transmission between patients. It also shows high rates of antifungal resistance, with approximately 90% of isolates resistant to fluconazole. The World Health Organization classified C. auris as a “critical priority” pathogen in 2022.

Cryptococcus gattii expanded from tropical regions into temperate areas like the Pacific Northwest, while Aspergillus species increasingly show resistance to antifungal medications. The rising prevalence of fungal pathogens correlates with increased use of immunosuppressive therapies and broader application of antifungal agents in agriculture and medicine.

Parasitic emerging pathogens

Parasitic organisms, including protozoa and helminths, also contribute to the emerging pathogen landscape. Plasmodium knowlesi, a malaria parasite primarily affecting monkeys, has emerged as a significant human pathogen in Southeast Asia. This zoonotic parasite can cause severe disease and has become a leading cause of malaria in some regions.

Other emerging parasitic threats include Cryptosporidium species causing waterborne disease outbreaks and Cyclospora cayetanensis linked to contaminated produce. These parasites often emerge when changes in food production, water treatment, or environmental conditions create new transmission opportunities.

Prions as unique emerging threats

Prions represent a unique category of emerging pathogens. These misfolded proteins cause devastating neurological diseases like Creutzfeldt-Jakob disease and its variant form linked to bovine spongiform encephalopathy (mad cow disease). Though rare, prion diseases are invariably fatal and pose unique challenges for diagnosis and containment.

Why pathogens emerge

Multiple interconnected factors drive pathogen emergence. Human encroachment on nature creates opportunities for infectious agents to emerge, jump species, and potentially adapt to spread among humans. Population growth and urbanization concentrate people in dense settings where pathogens spread more easily. Global travel networks enable rapid international spread of diseases that once remained geographically contained.

Changes in land use and agricultural practices alter the human-animal interface, increasing contact between people and wildlife reservoirs of disease. Modern food production systems, including intensive animal farming, can amplify pathogen transmission and select for more virulent strains. Climate change affects disease ecology by altering vector distributions and creating conditions favorable for pathogen survival and transmission.

Antimicrobial resistance emerges when antibiotics, antifungals, and antivirals are overused in human medicine and agriculture. This selective pressure allows resistant organisms to survive and proliferate. Declining vaccination coverage can also lead to resurgence of previously controlled diseases, as seen with measles outbreaks in areas with low immunization rates.

The public health challenge

Zoonotic pathogens are twice as likely to be emerging or reemerging compared to non-zoonotic pathogens, highlighting the critical importance of monitoring animal populations and human-animal interactions. However, only a small minority of emerging pathogens prove capable of causing major human epidemics. The challenge lies in identifying which emerging threats require intensive public health response versus routine surveillance.

Effective control of emerging pathogens requires multidisciplinary approaches combining surveillance, rapid diagnostic capabilities, infection control measures, and coordinated international responses. Public health infrastructure must remain vigilant for unexpected threats while addressing known emerging diseases. The development of new antimicrobials, vaccines, and treatment strategies must keep pace with evolving pathogens.

What do you think? How can healthcare systems better balance preparedness for unknown emerging pathogens with resources needed to address current infectious disease challenges? What role should international cooperation play in detecting and responding to emerging pathogen threats before they become global crises?

References

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Table of Contents

What defines an emerging pathogen 🔗

Viral emerging pathogens 🔗

Arboviruses as emerging threats 🔗

Bacterial emerging pathogens 🔗

Fungal emerging pathogens 🔗

Parasitic emerging pathogens 🔗

Prions as unique emerging threats 🔗

Why pathogens emerge 🔗

The public health challenge 🔗