From viral infection to emission: host barriers shaping airborne transmission.

Coughing, speaking, and even quiet breathing can release virus-laden particles into the air, triggering transmission events that spread through households and communities. Understanding why some respiratory viruses transmit so efficiently, and under which biological or environmental conditions this occurs, is central to anticipating future outbreaks. In this review, we outline the biological and physical factors that shape respiratory virus transmission, highlighting how the site of replication, mechanisms of droplet generation, and environmental conditions influence airborne spread. We examine how viral traits such as receptor usage, tissue tropism, and innate immune evasion intersect with airway physiology to determine the size, composition, and infectiousness of exhaled particles. We lastly draw attention to key gaps in current knowledge, particularly regarding the host and environmental factors that govern both the release and uptake of virions. Filling these gaps will allow us to more accurately assess transmission risk and implement targeted approaches to reduce spread.