With over 600 known human viruses and new ones emerging, experts highlight how climate change, globalization, and zoonotic spillover are accelerating viral threats—are we ready for the next pandemic?

Invited Review: Human viruses: An ever-increasing list. Image Credit: Lightspring / Shutterstock

In a recent review published in the journal Virology, researchers examined the increasing diversity of human viruses, their epidemiological significance, and preventive strategies to mitigate emerging threats.

Background

Did you know that nearly half of the world’s population is at risk of contracting dengue virus (DENV), a mosquito-borne illness with a confirmed 1200% increase in reported cases over the past two decades, from 0.5 million cases in 2000 to 6.5 million in 2023? However, the estimated true incidence is up to 400 million cases annually.

Viral diseases have shaped human history, with outbreaks causing widespread devastation, from the Black Death to the coronavirus disease 2019 (COVID-19) pandemic. While some viruses, such as smallpox, have been eradicated through vaccination, others persist, adapt, and continue to pose public health threats.

The advent of metagenomic analyses has unveiled numerous novel viruses, yet their pathogenic potential remains largely unknown. The ability of viruses to mutate and jump between species—often facilitated by globalization, climate change, and human encroachment into wildlife habitats—has led to the continuous emergence of new infectious threats.

Aedes-borne viruses, including DENV and Zika virus (ZIKV), have demonstrated the rapid spread of vector-borne diseases due to climate shifts. Moreover, viruses of zoonotic origin, such as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), have highlighted the unpredictable nature of viral spillover events. Notably, bats serve as major reservoirs for several zoonotic viruses, including SARS-CoV-2, Ebola virus (EBOV), Marburg virus (MARV), and Nipah virus (NiV), while livestock intermediaries such as pigs (for NiV) and camels (for MERS-CoV) facilitate viral transmission to humans.

Further research is needed to anticipate and mitigate the risks associated with emerging viruses before they escalate into global crises.

Human Virus Diversity and Evolution

Over the past century, the number of known human viruses has expanded significantly. Currently, more than 600 viruses across 30 virus families have been identified as human pathogens, including arboviruses (arthropod-borne viruses), zoonotic viruses, and human-adapted viruses.

Arboviruses, such as chikungunya virus (CHIKV) and West Nile virus (WNV), primarily spread through mosquito vectors, while zoonotic viruses, including Ebola virus (EBOV) and Lassa virus (LASV), originate from animal reservoirs. Many of these viruses exhibit high adaptability, enabling them to exploit new hosts and transmission pathways.

The continuous evolution of viruses is driven by genetic recombination, mutation, and selection pressure. For example, the emergence of new influenza A virus strains due to antigenic shift has led to multiple pandemics.

Similarly, the rapid evolution of SARS-CoV-2 has resulted in highly transmissible variants, complicating containment efforts. The interplay between viral adaptation and human immunity underscores the necessity for ongoing surveillance and vaccine development.

Transmission Pathways and Impact on Individuals and Communities

Human viruses spread through diverse mechanisms, including direct contact, airborne transmission, vector-borne routes, and zoonotic spillover. Respiratory viruses, such as measles virus (MeV) and SARS-CoV, transmit through aerosolized droplets, making them highly contagious. Bloodborne viruses, including human immunodeficiency virus (HIV) and hepatitis B virus (HBV), pose risks through unsafe medical practices and unprotected sexual contact.

The societal and economic impact of viral outbreaks is profound. Beyond the immediate health effects, individuals and communities face long-term consequences, such as loss of income, overwhelmed healthcare systems, and disruptions in education.

Vector-borne viruses, such as DENV and yellow fever virus (YFV), have triggered economic downturns in affected regions by hindering tourism and trade. Similarly, the COVID-19 pandemic exposed vulnerabilities in global supply chains and healthcare infrastructure, demonstrating the far-reaching consequences of viral diseases.

Additionally, Oropouche virus (OROV), an emerging arbovirus, has been expanding rapidly in Latin America, with over 10,000 cases reported since December 2023. Its recent outbreaks underscore the need for enhanced surveillance and response strategies against newly emerging vector-borne threats.

Global Health Impact and Emerging Threats

Viral epidemics and pandemics impose substantial burdens on healthcare systems and economies. The COVID-19 pandemic, which has resulted in over seven million deaths globally, underscores the devastating impact of novel viruses.

Similarly, the resurgence of DENV, which now threatens nearly half of the world’s population, highlights the challenge of controlling vector-borne diseases.

Beyond immediate health impacts, emerging viruses contribute to long-term consequences, including chronic disease and economic instability. Several oncogenic viruses, including human papillomavirus (HPV), Epstein-Barr virus (EBV), hepatitis B virus (HBV), hepatitis C virus (HCV), and human T-lymphotropic virus 1 (HTLV-1), are known to cause cancers such as cervical cancer, liver cancer, and adult T-cell leukemia.

The integration of genomic surveillance, public health interventions, and global cooperation is crucial for mitigating future viral threats.

Frank Macfarlane Burnet, Nobel laureate and pioneering virologist, described viruses as: “A virus is not an individual organism in the ordinary sense of the term, but something which could almost be called a stream of biological patterns.”

Prevention and Control Strategies

Effective viral prevention strategies encompass vaccination, vector control, public health measures, and early detection systems.

Vaccination remains the cornerstone of viral disease prevention, with successful programs eliminating smallpox and reducing poliovirus transmission. However, gaps in immunization coverage, vaccine hesitancy, and logistical challenges hinder immunization efforts in many regions. For example, the dengue vaccine (Dengvaxia) is only recommended for individuals who have already been exposed to DENV, limiting its widespread use.

Vector control measures, such as eliminating mosquito breeding sites and deploying genetically modified mosquitoes, have shown promise in curbing arboviral transmission. For zoonotic viruses, wildlife surveillance and improved biosecurity practices in livestock farming are critical.

Personal protective measures, such as hand hygiene, mask-wearing, and safe sexual practices, can significantly reduce viral spread.

International collaboration is essential for pandemic preparedness. The World Health Organization (WHO) has prioritized high-risk pathogens, including Disease X (an unidentified future pandemic threat), underscoring the need for proactive research and response strategies. Strengthening health infrastructure, enhancing diagnostic capabilities, and investing in antiviral development are imperative to combating viral threats.

Michael Osterholm, director of the Center for Infectious Disease Research and Policy, emphasized the unpredictability of avian influenza: “It’s really important to understand that no one—and I mean no one—knows what H5N1 influenza virus is going to do to the human population.”

Conclusions

Human viruses represent an ever-growing challenge due to their rapid evolution, diverse transmission pathways, and global health impact. The increasing interface between humans and viral reservoirs, driven by climate change, urbanization, and international travel, has heightened the risk of emerging infectious diseases.

The rapid expansion of vector-borne and zoonotic viruses necessitates enhanced surveillance, preventive strategies, and global cooperation to mitigate future outbreaks.

Vaccination, vector control, and public health interventions remain critical in reducing the burden of viral diseases. However, emerging viral mutations and limited vaccine availability for certain viruses, such as arboviruses and coronaviruses, pose ongoing challenges.

The COVID-19 pandemic has demonstrated the necessity for proactive pandemic preparedness, including genomic surveillance and rapid response frameworks.

“A virus is a piece of bad news wrapped in protein.” — Sir Peter Medawar, British immunologist and Nobel laureate.