In 2026, multiple expert groups (WHO, academic public health centers, vaccine alliances) have highlighted a cluster of infectious threats that may drive renewed concern, due to changing immunity patterns, low vaccination coverage, zoonotic spillover, and antimicrobial resistance. Key sources consistently point to respiratory viruses (influenza variants, COVID‑19), vaccine‑preventable childhood infections (measles, polio), emerging arboviruses (Oropouche), zoonotic influenza strains (H5N1, others), and tuberculosis as priority watch‑list pathogens [1][2][3][4][5][8].

Infections Most Likely to Trigger Renewed Concern

1. Influenza viruses (including avian strains such as H5N1)

• WHO and Gavi emphasize influenza viruses as ongoing top threats, with H5N1 avian influenza under particularly close watch due to its spread in birds and sporadic mammalian infections [1].

• Infectious disease experts note Influenza A as a perennial pandemic candidate given its wide animal reservoir and rapid mutation [2].

• Emerging analyses also flag influenza D as a potential future concern if human adaptation increases [4][5].

Why renewed concern?

• Waning population immunity and changing vaccine uptake.

• Potential for antigenic shift/reassortment leading to pandemic‑capable strains.

2. Measles

• Several 2026 disease‑watch summaries list measles as a top infection to watch in 2026 due to declining vaccination rates and resurgent outbreaks [3][8].

• Gaps in routine immunization in some regions risk large outbreaks with high morbidity, especially in malnourished or immunocompromised populations.

Why renewed concern?

• Highly contagious (R₀ often >10–15).

• Outbreaks can quickly strain health systems and signal erosion of immunization programs.

3. Polio (including vaccine-derived polioviruses)

• Experts highlight polio as another infection likely to resurface in public concern, particularly due to circulating vaccine-derived polioviruses (cVDPV) in under‑immunized areas [3].

Why renewed concern?

• Reversion of attenuated vaccine strains in low-coverage settings.

• Complexity of eradication endgame, with risks heightened by global mobility.

4. COVID‑19 variants and “Disease X”

• While COVID‑19 is now endemic, 2026 outlooks list COVID variants and the broader concept of “Disease X” as continuing concerns [3].

• Simultaneous circulation of flu, RSV, COVID variants, and other respiratory viruses is causing high seasonal burdens and periodic spikes [7].

Why renewed concern?

• Immune escape variants may reduce vaccine effectiveness.

• Health systems face cumulative strain from multiple respiratory pathogens co‑circulating.

5. Mpox and other orthopoxviruses

• Regional experts include mpox (monkeypox) among viruses to watch in 2026, alongside avian flu and Oropouche [8].

• Clusters in under‑vaccinated groups and international travel maintain risk of renewed outbreaks, especially if surveillance or vaccine access falters.

6. Oropouche virus and other emerging arboviruses

• The Conversation and associated experts highlight Oropouche virus as a likely ongoing source of outbreaks in the Americas in 2026, spread by biting midges [5].

• Other arboviruses (e.g., dengue, Zika, chikungunya) remain significant regional concerns where vectors and climate conditions allow, though Oropouche is specifically called out as an “emerging” traveler risk [5].

Why renewed concern?

• Climate and ecological changes expanding vector ranges.

• Limited awareness and diagnostic capacity outside endemic regions.

7. Tuberculosis (TB)

• TB is described as the world’s top infectious killer and is “creeping back” in the US and other high‑income settings after hopes of eradication [9].

• Drug‑resistant TB strains and pandemic‑related disruptions to TB services worsen the outlook.

Why renewed concern?

• Rising incidence in some areas after years of decline.

• Growing burden of drug-resistant TB and persistent health inequities.

Counterarguments & Uncertainties

• Not all highlighted pathogens will cause large outbreaks every year; some may remain localized or largely controlled.

• Improved vaccination and surveillance efforts can mitigate risk if political will and resources are sustained.

• “Disease X” is a placeholder concept: the specific pathogen is unknown, but capacity‑building for broad preparedness can reduce surprise.

Implications for Health Systems

• Surveillance and diagnostics: Strengthen genomic and syndromic surveillance for influenza (including avian strains), SARS‑CoV‑2, and emerging arboviruses.

• Vaccination systems: Focus on restoring and expanding coverage for measles, polio, and COVID‑19 vaccines.

• High‑risk populations: Tailor communication and access for migrants, immunocompromised individuals, and communities with low trust in healthcare.

• Preparedness for overflow: Plan for seasonal surges when multiple respiratory pathogens co‑circulate, including bed capacity and staffing.