  Exhaled droplet and aerosol-mediated transmission of respiratory diseases,
including SARS-CoV-2, is exacerbated in poorly ventilated environments where
body heat-driven airflow prevails. Employing large-scale simulations, we reveal
how the human body heat can potentially spread pathogenic species between
occupants in a room. Morphological phase transition in airflow takes place as
the distance between human heat sources is varied which shapes novel patterns
of disease transmission: For sufficiently large distance, individual buoyant
plume creates a natural barrier, forming a ``thermal armour'' that blocks
suspension spread between occupants. However, for small distances, collective
effect emerges and thermal plumes condense into super-structure, facilitating
long-distance suspension transport via crossing between convection rolls. Our
quantitative analysis demonstrates that infection risk increases significantly
at critical distances due to collective behavior and phase transition. This
highlights the importance of maintaining reasonable social distancing indoors
to minimize viral particle transmission and offers new insights into the
critical behavior of pathogen spread.
