1.4 Our shared and evolving hazard landscape

Since 2020, the immunological hazard landscape has evolved at a rapid pace,^{Footnote 11} with more changes on the horizon as climate change fuels the rise of wildfire smoke, air pollution, and animal migration amplifying disease spread.^{Footnote 12} The last few years have seen an increase in the spread and severity of many infectious diseases.^{Footnote 13} Polio returned to North America in 2022,^{Footnote 14} while Canada lost its decades-long measles elimination status in November 2025.^{Footnote 15} After decades of steady decline, tuberculosis cases have surged,^{Footnote 16} while seasonal influenza in 2025–2026 proved to be more virulent and deadly than in previous years.^{Footnote 17} Canada experienced its first human case of H5N1 avian influenza in 2024, with public health officials stating that the illness has “pandemic potential.”^{Footnote 18} Hospitalization rates for common respiratory-transmitted diseases are up in Canada, even those for which vaccinations exist.^{Footnote 19}

Two factors in the evolving hazard landscape are that many people do not understand they are at risk,^{Footnote 20} and that many of these hazards spread through the air we share. It is well-established that many respiratory pathogens spread via invisible airborne particles (aerosols), which can linger in the air for long periods of time.^{Footnote 21} This highlights the need to address poor indoor air quality—an issue equally important for non-communicable diseases like asthma, chronic obstructive pulmonary disease (COPD), and seasonal allergies, which are also exacerbated by climate change.^{Footnote 22}

“For too long, indoor air has been the invisible crisis of our time. We spend nearly 90% of our lives indoors. . . . This is not only a health crisis; it is also an economic crisis. Poor air quality drains productivity, drives health costs and weakens our resilience to pandemics. . . . Protecting indoor air is as urgent as ensuring clean water or safe food. The solutions are within reach.”

• Jose Luis Castro, Director-General Special Envoy for Chronic Respiratory Diseases, World Health Organization^{Footnote 23}