  The Covid-19 pandemic has taken millions of lives, demonstrating the tragedy
and disruption of respiratory diseases, and how difficult they can be to
manage. However, there is still significant debate in the scientific community
as to which transmission pathways are most significant and how settings and
behaviour affect risk of infection, which all have implications for which
mitigation strategies are most effective. This study presents a general model
to estimate the rate of viral transfer between individuals, objects, and the
air. The risk of infection to individuals in a setting is then computed
considering the behaviour and interactions of individuals between themselves
and the environment in the setting, survival times of the virus on different
surface types and in the air, and mitigating interventions (ventilation, hand
disinfection, surface cleaning, etc.). The model includes discrete events such
as touch events, individuals entering/leaving the setting, and cleaning events.
  We demonstrate the model capabilities on three case studies to quantify and
understand the relative risk associated with the different transmission
pathways and the effectiveness of mitigation strategies in different settings.
The results show the importance of considering all transmission pathways and
their interactions, with each scenario displaying different dominant pathways
depending on the setting and behaviours of individuals therein. The flexible
model, which is freely available, can be used to quickly simulate the spread of
any respiratory virus via the modelled transmission pathways and the efficacy
of potential mitigation strategies in any enclosed setting by making reasonable
assumptions regarding the behaviour of its occupants. It is hoped that the
model can be used to inform sensible decision-making regarding viral infection
mitigations that are targeted to specific settings and pathogens.
